Feature: add disable_notification param

[empas]

Hi. Just tried to add silent param to your Message type and it works.

{
      key: 'notification',

      /**
       * Set disable_notification of the message
       * @param  {boolean} enable
       * @return {object} returns the message object
       */
      value: function preview() {
          var enable = arguments[0] === undefined ? true : arguments[0];

          this.properties.disable_notification = !enable;
          return this;
      }
}