Implement -andForwardToRealObject for class method mocks, option 2

Implement -andForwardToRealObject for class method mocks.  This version can no longer tell the user up front of illegal uses of andForwardToRealObject, but will instead throw the same error message later at runtime if the handler is actually used in an illegal situation.  But, it does not require the explicit use of -classMethod when mocking -- it should work as naturally as andForwardToRealObject does for partial mocks.

Either this pull request or #64 should be added, not both.  See that pull request for more discussion.