Install an event handler for the given event name
function (eventName, handler) {
    if (typeof(eventName) !== 'string') {
      throw new Error('Argument eventName should be a string');
    }
    if (typeof(handler) !== 'function') {
      throw new Error('Argument handler should be a function');
    }
    log('[Eventhandling] Adding event listener', eventName);
    this._validateEvent(eventName);
    this._handlers[eventName].push(handler);
  }