Integrated primaryKey extension to NSEntityDescription and refactored cache strategy

classes to eliminate issues with duplicated objects. closes #611, #612, #613, #618

* NSEntityDescription is now aware of the primaryKeyAttribute. Can be configured via
Interface Builder within Xcode or programatically.
* Added findByPrimaryKey: interface to the Core Data extensions.
* Relaxed dependencies on RKManagedObjectMapping across the system now that primaryKey is
available without a reference to the mapping.

Code/CoreData/NSManagedObject+ActiveRecord.h

@@ -22,7 +22,8 @@
 @end
 
 /**
- Extensions for NSManage
+ Provides extensions to NSManagedObject implementing a low-ceremony querying
+ interface.
  */
 @interface NSManagedObject (ActiveRecord)
 
@@ -102,6 +103,25 @@
  */
 - (BOOL)isNew;
 
+/**
+ Finds the instance of the receiver's entity with the given value for the primary key attribute
+ in the managed object context for the current thread.
+
+ @param primaryKeyValue The value for the receiving entity's primary key attribute.
+ @return The object with the primary key attribute equal to the given value or nil.
+ */
++ (id)findByPrimaryKey:(id)primaryKeyValue;
+
+/**
+ Finds the instance of the receiver's entity with the given value for the primary key attribute in
+ the given managed object context.
+
+ @param primaryKeyValue The value for the receiving entity's primary key attribute.
+ @param context The managed object context to find the instance in.
+ @return The object with the primary key attribute equal to the given value or nil.
+ */
++ (id)findByPrimaryKey:(id)primaryKeyValue inContext:(NSManagedObjectContext *)context;
+
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
 + (NSManagedObjectContext*)currentContext;