Introducing AFHTTPMethod in AFHTTPClient class:
- AFHTTPMethod: enum of AFDeleteHTTPMethod, AFGetHTTPMethod, AFHeadHTTPMethod, AFPostHTTPMethod, AFPutHTTPMethod
- 2 new request methods (requestWithHTTPMethod, multipartFormRequestWithHTTPMethod) take AFHTTPMethod as http method input (instead of NSString)
- hide 2 olds methods requestWithMethod, multipartFormRequestWithMethod (privates methods)
- method stringFromHTTPMethod: convert HTTPMethod to string