Allow custom scheme, use current version of AFOAuth1Client, manual authorization step, hide some implementation details

- Add support for custom URL-Schemes as OAuth callback.
- Use current version of AFOAuth1Client via cocoa pods instead of an old prefixed copy.
- Hide implementation details of XNGOAuthHandler.
- some DRYing

These changes also includes a first step to clean up OAuth dependencies.
The next step would be to use AFOAuth1Client as superclass of XNGApiClient and to get rid of the CTMOAuth dependency and some related XNGOAuthHandler methods.