getLabel(label, cb) {
      return this._request('GET', `/repos/${this.__repository}/labels/${label}`, null, cb);
   }
Get a label
@see https://developer.github.com/v3/issues/labels/#get-a-single-label
@param {string} label - the name of the label to fetch
@param {Requestable.callback} [cb] - will receive the label
@return {Promise} - the promise for the http request