createComment(comment, cb) {
      return this._request('POST', `/gists/${this.__id}/comments`, {body: comment}, cb);
   }
Comment on a gist
@see https://developer.github.com/v3/gists/comments/#create-a-comment
@param {string} comment - the comment to add
@param {Requestable.callback} [cb] - the function that receives the API result
@return {Promise} - the Promise for the http request