Add Protocol Buffer support

 `AFProtocolBufferSessionManager` is a subclass of `AFHTTPSessionManager` for sending Protocol Buffers over TCP.
 ## Serialization
 
 AFURLRequestSerialization and AFURLResponseSerialization is not used because they can not be uniquely set for the each endpoint request/response call.
 
 ## Example
 
 NSString * send:urlString = @"/api/user";
 
 UserRequest_Builder * builder = [UserRequest_Builder new];
 builder.email = email;
 builder.password = password;
 
 UserRequest * request = [builder build];
 
 AFProtocolBufferSessionManager * manager = self.sharedManager;
 
 NSURLSessionDataTask * task = [manager send: urlString
                                                                  request: request
                                                               response: [UserResponse class]
                                                                 success: success
                                                                    failure: failure];