code: function(event, callback) {
      if ('ready-success-fail'.split('-').indexOf(event) >= 0 && _typeof(callback) == 'function')
        _events[event] = callback;
    }
docs: Result: the function to get `Result` instance. The request result object.
  - xhrObj: the xhr obj which can get the xhr result.
  
  How to use it? if `r` is the instance of `Result`.
  var r_json = r.json();
  var status = r.status_code;
  var headers = r.headers;