added a Swiftmailer extension

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')
    );

src/Silex/Extension/SwiftmailerExtension.php

+<?php
+
+/*
+ * This file is part of the Silex framework.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Silex\Extension;
+
+use Silex\Application;
+use Silex\ExtensionInterface;
+
+class SwiftmailerExtension implements ExtensionInterface
+{
+    public function register(Application $app)
+    {
+        $app['swiftmailer.options'] = array_replace(array(
+            'host'       => 'localhost',
+            'port'       => 25,
+            'username'   => '',
+            'password'   => '',
+            'encryption' => null,
+            'auth_mode'  => null,
+        ), isset($app['swiftmailer.options']) ? $app['swiftmailer.options'] : array());
+
+        $app['mailer'] = $app->share(function () use ($app) {
+            $r = new \ReflectionClass('Swift_Mailer');
+            require_once dirname($r->getFilename()).'/../../swift_init.php';
+
+            return new \Swift_Mailer($app['swiftmailer.transport']);
+        });
+
+        $app['swiftmailer.transport'] = $app->share(function () use ($app) {
+            $transport = new \Swift_Transport_EsmtpTransport(
+                $app['swiftmailer.transport.buffer'],
+                array($app['swiftmailer.transport.authhandler']),
+                $app['swiftmailer.transport.eventdispatcher']
+            );
+
+            $transport->setHost($app['swiftmailer.options']['host']);
+            $transport->setPort($app['swiftmailer.options']['port']);
+            $transport->setEncryption($app['swiftmailer.options']['encryption']);
+            $transport->setUsername($app['swiftmailer.options']['username']);
+            $transport->setPassword($app['swiftmailer.options']['password']);
+            $transport->setAuthMode($app['swiftmailer.options']['auth_mode']);
+
+            return $transport;
+        });
+
+        $app['swiftmailer.transport.buffer'] = $app->share(function () {
+            return new \Swift_Transport_StreamBuffer(new \Swift_StreamFilters_StringReplacementFilterFactory());
+        });
+
+        $app['swiftmailer.transport.authhandler'] = $app->share(function () {
+            return new \Swift_Transport_Esmtp_AuthHandler(array(
+                new \Swift_Transport_Esmtp_Auth_CramMd5Authenticator(),
+                new \Swift_Transport_Esmtp_Auth_LoginAuthenticator(),
+                new \Swift_Transport_Esmtp_Auth_PlainAuthenticator(),
+            ));
+        });
+
+        $app['swiftmailer.transport.eventdispatcher'] = $app->share(function () {
+            return new \Swift_Events_SimpleEventDispatcher();
+        });
+
+        if (isset($app['swiftmailer.class_path'])) {
+            $app['autoloader']->registerPrefix('Swift_', $app['swiftmailer.class_path']);
+        }
+    }
+}