Support mock.create_autospec with spec_set=True

[bxm156]

The current models do not support auto speccing. If you attempt to use mock.create_autospec with spec_set=True, you will get

AttributeError: Mock object has no attribute '<attribute name here>'

I think this is due to the way we handle creating and setting properties in https://github.com/Yelp/bravado-core/blob/master/bravado_core/model.py. Not sure how easy or feasible a change this would be based on the current way of doing things. But it would be nice to have.