Major overhaul to the Core Data managed object identification and relationship connection support.

* Replaces primary key with `RKEntityIdentifier`
* Add support for use of compound keys for object identification
* Refactor `RKConnectionMapping` to `RKConnectionDescription` and add support for connecting with multiple attributes
* Clarify naming of representation key methods to better match naming conventions
* Add type transformation support for object identification
* Greatly expand test coverage for object identification
* Drop the `NSEntityDescription` category
* Simplify the `RKManagedObjectCaching` protocol
* Add compound key support to the Fetch Request and In Memory Cache implementations
* Replace Kiwi with Specta for tests where contexts are helpful for organization
* Rename `defaultValueForMissingAttribute` to `defaultValueForAttribute`

Code/ObjectMapping/RKValueTransformers.m

@@ -0,0 +1,53 @@
+//
+//  RKValueTransformers.m
+//  RestKit
+//
+//  Created by Blake Watters on 11/26/12.
+//  Copyright (c) 2012 RestKit. All rights reserved.
+//
+
+#import "RKValueTransformers.h"
+#import "RKMacros.h"
+
+// Implementation lives in RKObjectMapping.m at the moment
+NSDate *RKDateFromStringWithFormatters(NSString *dateString, NSArray *formatters);
+
+@implementation RKDateToStringValueTransformer
+
++ (Class)transformedValueClass
+{
+    return [NSDate class];
+}
+
++ (BOOL)allowsReverseTransformation
+{
+    return YES;
+}
+
+- (id)initWithDateToStringFormatter:(NSFormatter *)dateToStringFormatter stringToDateFormatters:(NSArray *)stringToDateFormatters
+{
+    self = [self init];
+    if (self) {
+        self.dateToStringFormatter = dateToStringFormatter;
+        self.stringToDateFormatters = stringToDateFormatters;
+    }
+    return self;
+}
+
+- (id)transformedValue:(id)value
+{
+    NSAssert(self.stringToDateFormatters, @"Cannot transform an `NSDate` to an `NSString`: stringToDateFormatters is nil");
+    RKAssertValueIsKindOfClass(value, [NSString class]);
+    return RKDateFromStringWithFormatters(value, self.stringToDateFormatters);
+}
+
+- (id)reverseTransformedValue:(id)value
+{
+    NSAssert(self.dateToStringFormatter, @"Cannot transform an `NSDate` to an `NSString`: dateToStringFormatter is nil");
+    RKAssertValueIsKindOfClass(value, [NSDate class]);
+    @synchronized(self.dateToStringFormatter) {
+        return [self.dateToStringFormatter stringForObjectValue:value];
+    }
+}
+
+@end