createKey(options, cb) {
      return this._request('POST', `/repos/${this.__fullname}/keys`, options, cb);
   }
Add a new deploy key to the repository
@see https://developer.github.com/v3/repos/keys/#add-a-new-deploy-key
@param {Object} options - the configuration describing the new deploy key
@param {Requestable.callback} cb - will receive the new deploy key
@return {Promise} - the promise for the http request