start(port) {
    this._initWebhook();
    this.app.set('port', port || 3000);
    this.server = this.app.listen(this.app.get('port'), () => {
      const portNum = this.app.get('port');
      console.log('BootBot running on port', portNum);
      console.log(`Facebook Webhook running on localhost:${portNum}${this.webhook}`);
    });
  }
Starts the express server on the specified port. Defaults port to 3000.
@param {Number} [port=3000]