Make CDRFake inherit from NSProxy

At present, any methods implemented by NSObject can't be stubbed or record invocations. Since NSObject implements the methods, calls are never forwarded, so calls never make it to CDRDoubleImpl.

Not really a problem with most of the NSObject protocol methods, but methods added by categories can be useful to stub/verify (`valueForKey:`, for example).

Making CDRFake inherit from NSProxy allows stubbing the useful methods implemented by NSObject.