Mocktail support (+ Subspec split)

• Added support for stubs written in the Mocktail format (@JinlianWang, #108)
• The OHHTTPStubs spec has been splitted into multiple subspecs:
  • The default subspec (used when you simply use pod 'OHHTTPStubs' in your Podfile) contains the subspecs Core, NSURLSession, JSON & OHPathHelpers (so that it matches the features that most people use).
  • Other optional subspecs are HTTPMessage and Mocktail (which are opt-in because used by much less people). If you want to use them, you'll need to request them explicitly in your Podfile using pod 'OHHTTPStubs/Mocktail for example.
• The iOS Unit Tests are now also run for the framework as well as for the static library, to ensure the tests pass in both contexts (because frameworks sometimes introduce subtleties like when using NSBundle, so it's worth testing in that context too)