added RestKit pod and refactored OAuth implementation

Hi Jeyben! I messaged you a few weeks back about using RestKit with this project. RestKit requires AFNetworking 1.3, while you use AFNetworking >= 2.0. I rewrote parts of the HTTP Client to work without the operation manager, which allows me to use the lower version of AFNetworking. I thought this might be useful for anyone else in a similar situation. Maybe as a separate branch of your project just for folks who have already committed to using the RestKit pod in their project as I had. Let me know what your thoughts are!