add bower assets

as described in the symfony cookbook http://symfony.com/doc/current/cookbook/frontend/bower.html

Symfony.gitignore

@@ -26,6 +26,9 @@
 /web/bundles/
 /web/uploads/
 
+# Assets managed by Bower
+/web/assets/vendor/
+
 # PHPUnit
 /app/phpunit.xml
 /phpunit.xml