AFNetworking 2.0

This adds support for AFNetworking 2.0.

Current steps needed to get this merged:

- [ ] Test XAuth
- [ ] Add error handling for common errors [see previous implementation](https://github.com/pietbrauer/XNGAPIClient/blob/master/XNGAPIClient/XNGJSONRequestOperation.m#L33-L46)
This call should be removed for this major release.