code: private void post(final String endpoint, final Class responseClass, final KickflipCallback cb) {
          post(endpoint, null, responseClass, cb);
      }
docs: Do a POST Request, creating a new user if necessary
  
  @param endpoint      Kickflip endpoint. e.g /user/new
  @param responseClass Class of the expected response
  @param cb            Callback that will receive an instance of responseClass