added a Session extension

src/Silex/Extension/SessionExtension.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;
+use Symfony\Component\HttpFoundation\SessionStorage\NativeSessionStorage;
+use Symfony\Component\HttpFoundation\Session;
+use Symfony\Component\HttpKernel\Events as HttpKernelEvents;
+
+class SessionExtension implements ExtensionInterface
+{
+    private $app;
+
+    public function register(Application $app)
+    {
+        $this->app = $app;
+
+        $app['session'] = $app->share(function () use ($app) {
+            return new Session($app['session_storage']);
+        });
+
+        $app['session_storage'] = $app->share(function () {
+            return new NativeSessionStorage();
+        });
+
+        $app['dispatcher']->addListener(HttpKernelEvents::onCoreRequest, $this, -255);
+    }
+
+    public function onCoreRequest($event)
+    {
+        $request = $event->getRequest();
+        $request->setSession($this->app['session']);
+
+        // starts the session if a session cookie already exists in the request...
+        if ($request->hasSession()) {
+            $request->getSession()->start();
+        }
+    }
+}