added an extension for the Symfony2 Validator component

src/Silex/Extension/ValidationExtension.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\Validator\Validator;
+use Symfony\Component\Validator\Mapping\ClassMetadataFactory;
+use Symfony\Component\Validator\Mapping\Loader\StaticMethodLoader;
+use Symfony\Component\Validator\ConstraintValidatorFactory;
+
+class ValidationExtension implements ExtensionInterface
+{
+    public function register(Application $app)
+    {
+        $app['validator'] = $app->share(function () use ($app) {
+            return new Validator(
+                $app['validator.mapping.class_metadata_factory'],
+                $app['validator.validator_factory']
+            );
+        });
+
+        $app['validator.mapping.class_metadata_factory'] = $app->share(function () use ($app) {
+            return new ClassMetadataFactory(new StaticMethodLoader());
+        });
+
+        $app['validator.validator_factory'] = $app->share(function () {
+            return new ConstraintValidatorFactory();
+        });
+
+        if (isset($app['validation.class_path'])) {
+            $app['autoloader']->registerNamespace('Symfony\\Component\\Validation', $app['validation.class_path']);
+        }
+    }
+}