public static function getAccessToken($client_id, $code, $redirect_uri,
            $client_secret=NULL) {
        $full_url = Config::$MONEY_URL . "/oauth/token";
        return self::sendRequest($full_url, array(
            "code" => $code,
            "client_id" => $client_id,
            "grant_type" => "authorization_code",
            "redirect_uri" => $redirect_uri,
            "client_secret" => $client_secret
        ));
    }