public static function button($url, $params = array())
    {
        // configuration
        $config = self::config();

        // no configuration file
        if (!$config) {
            return null;
        }

        // Params
        $color = (isset($params['color']) && $params['color'] == "white") ? "white" : "blue";
        $style = (isset($params['style']) && $params['style'] == "button") ? "button" : "flat";

        return '<script type="text/javascript" src="https://clef.io/v3/clef.js" class="clef-button" data-app-id="'.$config['app_id'].'" data-color="'.$color.'" data-style="'.$style.'" data-redirect-url="'. $url . '"></script>';
    }