listLabels(options, cb) {
      return this._request('GET', `/repos/${this.__repository}/labels`, options, cb);
   }
List the labels for the repository
@see https://developer.github.com/v3/issues/labels/#list-all-labels-for-this-repository
@param {Object} options - filtering options
@param {Requestable.callback} [cb] - will receive the array of labels
@return {Promise} - the promise for the http request