Change the RKMappingSourceObject superclass from NSProxy to NSObject.  Should function basically the same way, but performance tests in #2065 showed an improvement for some unknown reason (test application never invoked forwardInvocation:, so it has nothing to do with that).  Implemented some NSObject protocol methods to make sure they get forwarded to the object, and implemented -forwardingTargetForSelector: which is a much faster forwarding mechanism in case it does get used.  Also must implement rk_classForPropertyAtKeyPath: for unit tests to pass since that was no longer being forwarded.  Changed self.object references to _object to avoid additional method call; may have squeezed a tiny bit more performance with that.
Merge pull request #2085 from carllindberg/2065-RKMappingSourceObjectSuperclass

Change the RKMappingSourceObject superclass to NSObject.