It is now possible to create partial mocks for proxies.

Changes entail:

1. Ensuring that partial mocks provide signatures for stubbed methods, rather than
relying on the real objects' implementations of -methodSignatureForSelector:
(which mislead, for real objects which are proxies).
2. Allowing real objects to handle -forwardInvocation: when partial mocks
do not.
Fix -andForwardToRealObject where the real object is itself a partial mock.
Using an OCMExpectationSequencer, it is now possible to verify the order of messages
received by multiple mocks.