create(gist, cb) {
      return this._request('POST', '/gists', gist, cb)
         .then((response) => {
            this.__id = response.data.id;
            return response;
         });
   }
Create a new gist.
@see https://developer.github.com/v3/gists/#create-a-gist
@param {Object} gist - the data for the new gist
@param {Requestable.callback} [cb] - will receive the new gist upon creation
@return {Promise} - the Promise for the http request