Porting Over Some Changes

Hi @wiistriker,

Thanks again for making `AFJSONRPCClient`. I recently had a reason to use it, and found it to be pretty well-suited to my problem.

In the process of working with it, I ended up refactoring the implementation to make better-use of what `AFHTTPClient` provides. I also reformatted some things and added a CocoaPod `.podspec` file.

Feel free to merge this upstream if you'd like.