Move boot sequence outside of Server Element

Add boot sequence in the app’s `index.php`, e.g.:

    $auth = \attitude\Elements\Auth_Element::instance($_SERVER['Authorization']);

Then later use the Server_Element::setAuth() method to inject dependency:

    try {
        $server = Server_Element::instance();
        $server->setAuth($auth);

        $server->respond();
    } catch (HTTPException $e) {
        if (is_callable(array($e, 'header'))) {
            $e->header();

            echo 'false';
        } else {
            echo $e->getMessage();
        }
    }