Remove a previously installed event handler
function (eventName, handler) {
    this._validateEvent(eventName);
    var hl = this._handlers[eventName].length;
    for (var i=0;i<hl;i++) {
      if (this._handlers[eventName][i] === handler) {
        this._handlers[eventName].splice(i, 1);
        return;
      }
    }
  }