code: function(method, url, body, onsuccess, onfail) {
      _method = method;
      _url = url;
      _body = body;
      on('success', onsuccess);
      on('fail', onfail);
      return _request();
    }
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;