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/RKFetchRequestManagedObjectCache.h

@@ -0,0 +1,19 @@
+//
+//  RKFetchRequestManagedObjectCache.h
+//  RestKit
+//
+//  Created by Jeff Arena on 1/24/12.
+//  Copyright (c) 2012 RestKit. All rights reserved.
+//
+
+#import "RKManagedObjectMappingCache.h"
+
+/**
+ Provides a simple managed object cache strategy in which every request for an object
+ is satisfied by dispatching an NSFetchRequest against the Core Data persistent store.
+ Performance can be disappointing for data sets with a large amount of redundant data
+ being mapped and connected together, but the memory footprint stays flat.
+ */
+@interface RKFetchRequestManagedObjectCache : NSObject <RKManagedObjectCacheing>
+
+@end