listComments(cb) {
      return this._requestAllPages(`/gists/${this.__id}/comments`, null, cb);
   }
List the gist's comments
@see https://developer.github.com/v3/gists/comments/#list-comments-on-a-gist
@param {Requestable.callback} [cb] - will receive the array of comments
@return {Promise} - the promise for the http request