forUsers(options, cb) {
      return this._search('users', options, cb);
   }
Search for users
@see https://developer.github.com/v3/search/#search-users
@param {Search.Params} [options] - additional parameters for the search
@param {Requestable.callback} [cb] - will receive the results of the search
@return {Promise} - the promise for the http request