Doubles now support classes that forward messages.

- For spies, classes must implement -respondsToSelector: and
  -methodSignatureForSelector:
- For fakes, classes must implement +instancesRespondToSelector: and
  +instanceMethodSignatureForSelector:
Better stub_method error if signature not found

- Previously we'd only check if the object responds to the selector
  which led to a crash when trying to verify arugment/return types
  because this requires the method signature
Better error for unrecognized calls sent to spies
Merge pull request #98 from pivotal/fix-spy_on-breaking-forwarding

spy_on no longer breaks forwarding by spy subject