introduced AFNetworking-2.0 support

I've added support for AFNetworking 2.0. The architecture is still work in progress though. It needs more clean choice between url handling method. I guess it's Roman's choice as he is the owner.

I've added AFXAuthRequestSerializer class which is responsible for adding auth headers to requests. 

As of AF2.0 AFHTTPClient is gone, and 2 new classes were introducted: AFHTTPSessionManager and AFHTTPRequestOperationManager. Changes on XAuth scope are the same for both classes.