createLabel(labelData, cb) {
      return this._request('POST', `/repos/${this.__repository}/labels`, labelData, cb);
   }
Create a new label
@see https://developer.github.com/v3/issues/labels/#create-a-label
@param {Object} labelData - the label definition
@param {Requestable.callback} [cb] - will receive the object representing the label
@return {Promise} - the promise for the http request