code: function(events, handler) {
          if (events === undefined) {
              return;
          }
          if (handler === undefined) {
              return;
          }
  
          var handlers = this.handlers;
          each(splitStr(events), function(event) {
              handlers[event] = handlers[event] || [];
              handlers[event].push(handler);
          });
          return this;
      }
docs: bind event
  @param {String} events
  @param {Function} handler
  @returns {EventEmitter} this