Usage:

    $app->register(new SwiftmailerExtension(), array(
        'swiftmailer.options' => array(
            'host'       => 'smtp.gmail.com',
            'port'       => 465,
            'encryption' => 'ssl',
            'auth_mode'  => 'login',
            'username'   => 'xxxxxx',
            'password'   => 'yyyyyy',
        )
    ));

    $app['mailer']->send($app['mailer']
        ->createMessage()
        ->setFrom('foo@example.com')
        ->addTo('bar@example.com')
        ->setSubject('Foo Title')
        ->setBody('Foo Body')
    );