Reworked Brendan Ribera's contributions around time zone handling to eliminate the use of transient

NSDateFormatters, added a preferredDateFormatter for use when serializing dates to strings,
replaced the use of the description method for date encoding to strings with invocation of the
preferredDateFormatter, added new attribute transformation strategy from NSDate -> NSString properties
(also using the preferred date formatter), and provided customization support for date handling globally
and on a per-mapping basis. closes #200, closes #313, closes #309, closes #308

Code/ObjectMapping/RKObjectMapping.h

@@ -33,13 +33,14 @@ relationship. Relationships are processed using an object mapping as well.
  */
 @interface RKObjectMapping : NSObject <RKObjectMappingDefinition> {
     Class _objectClass;
-    NSMutableArray* _mappings;
-    NSMutableArray* _dateFormatStrings;
-    NSString* _rootKeyPath;
+    NSMutableArray *_mappings;
+    NSString *_rootKeyPath;
     BOOL _setDefaultValueForMissingAttributes;
     BOOL _setNilForMissingRelationships;
     BOOL _forceCollectionMapping;
     BOOL _performKeyValueValidation;
+    NSArray *_dateFormatters;
+    NSDateFormatter *_preferredDateFormatter;
 }
 
 /**
@@ -121,12 +122,32 @@ relationship. Relationships are processed using an object mapping as well.
  */
 @property (nonatomic, assign) BOOL forceCollectionMapping;
 
+
 /**
- An array of date format strings to apply when mapping a
- String attribute to a NSDate property. Each format string will be applied
- until the date formatter does not return nil.
+ An array of NSDateFormatter objects to use when mapping string values
+ into NSDate attributes on the target objectClass. Each date formatter
+ will be invoked with the string value being mapped until one of the date
+ formatters does not return nil.
+ 
+ Defaults to the application-wide collection of date formatters configured via:
+ [RKObjectMapping setDefaultDateFormatters:]
+ 
+ @see [RKObjectMapping defaultDateFormatters]
  */
-@property (nonatomic, retain) NSMutableArray* dateFormatStrings;
+@property (nonatomic, retain) NSArray *dateFormatters;
+
+/**
+ The NSDateFormatter instance for your application's preferred date
+ and time configuration. This date formatter will be used when generating
+ string representations of NSDate attributes (i.e. during serialization to
+ URL form encoded or JSON format).
+ 
+ Defaults to the application-wide preferred date formatter configured via:
+ [RKObjectMapping setPreferredDateFormatter:]
+ 
+ @see [RKObjectMapping preferredDateFormatter]
+ */
+@property (nonatomic, retain) NSDateFormatter *preferredDateFormatter;
 
 /**
  Returns an object mapping for the specified class that is ready for configuration
@@ -416,3 +437,78 @@ relationship. Relationships are processed using an object mapping as well.
 - (Class)classForProperty:(NSString*)propertyName;
 
 @end
+
+/////////////////////////////////////////////////////////////////////////////
+
+/**
+ Defines the inteface for configuring time and date formatting handling within RestKit
+ object mappings. For performance reasons, RestKit reuses a pool of date formatters rather
+ than constructing them at mapping time. This collection of date formatters can be configured
+ on a per-object mapping or application-wide basis using the static methods exposed in this
+ category.
+ */
+@interface RKObjectMapping (DateAndTimeFormatting)
+
+/**
+ Returns the collection of default date formatters that will be used for all object mappings
+ that have not been configured specifically.
+ 
+ Out of the box, RestKit initializes the following default date formatters for you in the
+ UTC time zone:
+    * yyyy-MM-dd'T'HH:mm:ss'Z'
+    * MM/dd/yyyy
+ 
+ @return An array of NSDateFormatter objects used when mapping strings into NSDate attributes
+ */
++ (NSArray *)defaultDateFormatters;
+
+/**
+ Sets the collection of default date formatters to the specified array. The array should
+ contain configured instances of NSDateFormatter in the order in which you want them applied
+ during object mapping operations.
+ 
+ @param dateFormatters An array of date formatters to replace the existing defaults
+ @see defaultDateFormatters
+ */
++ (void)setDefaultDateFormatters:(NSArray *)dateFormatters;
+
+/**
+ Adds a date formatter instance to the default collection
+ 
+ @param dateFormatter An NSDateFormatter object to append to the end of the default formatters collection
+ @see defaultDateFormatters
+ */
++ (void)addDefaultDateFormatter:(NSDateFormatter *)dateFormatter;
+
+/**
+ Convenience method for quickly constructing a date formatter and adding it to the collection of default
+ date formatters
+ 
+ @param dateFormatString The dateFormat string to assign to the newly constructed NSDateFormatter instance
+ @param nilOrTimeZone The NSTimeZone object to configure on the NSDateFormatter instance. Defaults to UTC time.
+ @result A new NSDateFormatter will be appended to the defaultDateFormatters with the specified date format and time zone
+ @see NSDateFormatter
+ */
++ (void)addDefaultDateFormatterForString:(NSString *)dateFormatString inTimeZone:(NSTimeZone *)nilOrTimeZone;
+
+/**
+ Returns the preferred date formatter to use when generating NSString representations from NSDate attributes.
+ This type of transformation occurs when RestKit is mapping local objects into JSON or form encoded serializations
+ that do not have a native time construct.
+ 
+ Defaults to a date formatter configured for the UTC Time Zone with a format string of "yyyy-MM-dd HH:mm:ss Z"
+ 
+ @return The preferred NSDateFormatter to use when serializing dates into strings
+ */
++ (NSDateFormatter *)preferredDateFormatter;
+
+/**
+ Sets the preferred date formatter to use when generating NSString representations from NSDate attributes.
+ This type of transformation occurs when RestKit is mapping local objects into JSON or form encoded serializations
+ that do not have a native time construct.
+ 
+ @param dateFormatter The NSDateFormatter to configured as the new preferred instance
+ */
++ (void)setPreferredDateFormatter:(NSDateFormatter *)dateFormatter;
+
+@end