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/ObjectMapping/RKDynamicObjectMapping.h

@@ -27,12 +27,12 @@
 @protocol RKDynamicObjectMappingDelegate <NSObject>
 
 @required
-- (RKObjectMapping*)objectMappingForData:(id)data;
+- (RKObjectMapping *)objectMappingForData:(id)data;
 
 @end
 
 #ifdef NS_BLOCKS_AVAILABLE
-typedef RKObjectMapping*(^RKDynamicObjectMappingDelegateBlock)(id);
+typedef RKObjectMapping *(^RKDynamicObjectMappingDelegateBlock)(id);
 #endif
 
 /**

Code/ObjectMapping/RKObjectManager.h

@@ -366,7 +366,7 @@ typedef enum {
  For example:
     
     - (void)loadObjectUsingBlockExample {
-        [[RKObjectManager sharedManager] loadObjectsAtResourcePath:@"/monkeys.json" delegate:self block:^(RKObjectLoader* loader) {
+        [[RKObjectManager sharedManager] loadObjectsAtResourcePath:@"/monkeys.json" usingBlock:^(RKObjectLoader* loader) {
             loader.objectMapping = [[RKObjectManager sharedManager].mappingProvider objectMappingForClass:[Monkey class]];
         }];
     }