private static function connection()
    {
        if (!self::$connection) {
            self::$connection = new Connection();
            if (self::$client_id) {
                self::$connection->authenticateOauth(self::$client_id, self::$auth_token);
            } else {
                self::$connection->authenticateBasic(self::$username, self::$api_key);
            }
        }

        return self::$connection;
    }