Fix for warning emitted due to elimination of findOrCreateInstanceOfEntity:. Search Word functionality restored. closes #584

* Updates to the Core Data layer such that NSManagedObjectContexts now have a reference to the managed object store
they belong to.
* NSManagedObject instances can now return the managed object store they belong to.
* Relaxed the coupling to the sharedManager present within the RKSearchableManagedObject class.
* Expanded documentation of RKSearchableManagedObject

Code/CoreData/NSManagedObject+RKAdditions.h

@@ -0,0 +1,20 @@
+//
+//  NSManagedObject+RKAdditions.h
+//  RestKit
+//
+//  Created by Blake Watters on 3/14/12.
+//  Copyright (c) 2012 RestKit. All rights reserved.
+//
+
+#import <CoreData/CoreData.h>
+
+@class RKManagedObjectStore, RKManagedObjectMapping;
+
+@interface NSManagedObject (RKAdditions)
+
+/**
+ The receiver's managed object store.
+ */
+- (RKManagedObjectStore *)managedObjectStore;
+
+@end