Afnetworking 2.x

This is a modification of the code to work with AFNetworking 2.x
- Renamed AFHTTPClient category to AFHTTPRequestOperationManager to support change
- Renamed all convenience methods in AFHTTPRequestOperationManager (e.g rac_getPath -> rac_GET)
- Renamed AFURLConnection category to AFHTTPRequestOperation since it was exclusif to this class

Other modifications
- Unified AFHTTPClient and AFHTTPRequestOperation signal handling in AFHTTPRequestOperation rac_overrideHTTPCompletionBlock:
- Integrated the Podspec from the CocoaPods repos
- Replaced all submodules dependencies by a PodFile for the example