Update OAuthSDK to AFNetworking 2


The OAuthSDK requires AFNetworking 1.3, which is a problem for me because my existing project uses AFNetworking 2.5. [I'm not the only one with this issue](http://stackoverflow.com/questions/23760403/yammer-does-not-use-the-latest-version-of-afnetworking).

I've updated the OAuthSDK to AFNetworking 2.5 with minimal, non-breaking changes. 

Other things it might be a good idea to update:

- Change `getPath:parameters:success:failure:` and `postPath:parameters:success:failure:` methods in YMHTTPClient.h to match AFNetworking 2 methods (`GET:parameters:success:failure:` & `POST:parameters:success:failure:`).
- Rename the `YMHTTPClient` class and its `httpClient` property. This class is a wrapper for `AFHTTPClient` which does not exist in AFNetworking 2.

Thoughts?