Support mocking on BasicObject-subclassed proxies.

Created by: myronmarston

This is my first pass at the solution we talked about in #116.

We talked about adding a config API that would mix RSpec::Mocks::Methods into additional classes, but I'm leaning towards not adding that, and instead just documenting that users can include RSpec::Mocks::Methods in classes that need it. We don't already have a configuration object in rspec-mocks and it seemed overly heavy-weight to add it just so we can have an option that will mix a module into a class...when our users can mix the module into their desired class on their own.

i'm open to adding it if others think it's important.