code: function(err) {
          if (err) this._onError(err);
          if (this._continueErrors || !err) {
              var args = [].slice.call(arguments);
              if (this._callbacks.length > 0) {
                  this._isQueueRunning = true;
                  var cb = this._callbacks.shift();
                  cb = cb.bind(this);
                  args = args.slice(1);
                  args.push(this.next);
                  cb.apply(this, args);
  
              } else {
                  this._isQueueRunning = false;
                  this.start = (function() {
                      this.start = null;
                      this.next.apply(this, args);
                  }).bind(this);
  
              }
          }
          return this;
      }
docs: advance to next cb