isStarred(cb) {
      return this._request204or404(`/gists/${this.__id}/star`, null, cb);
   }
Check if a gist is starred by the user.
@see https://developer.github.com/v3/gists/#check-if-a-gist-is-starred
@param {Requestable.callback} [cb] - will receive true if the gist is starred and false if the gist is not starred
@return {Promise} - the Promise for the http request