createRef(options, cb) {
      return this._request('POST', `/repos/${this.__fullname}/git/refs`, options, cb);
   }
Create a reference
@see https://developer.github.com/v3/git/refs/#create-a-reference
@param {Object} options - the object describing the ref
@param {Requestable.callback} [cb] - will receive the ref
@return {Promise} - the promise for the http request