Added some Doctrine file caching for prod env

Since this web site will live on a shared hosting environment, we have no APC, Redis nor memcache... But because the shared hosting env performances are pretty hostile for a heavy framework like Symfony, we cant' afford to line with no data cache.
--> Let's use custom a file system cache! (not as efficient as a memory cache, but better than nothing)

* added "Doctrine Cache" Bundle
* enabled Http Cache in our prod front controller "app.php"
* added a custom "app/config/doctrine_cache.yml" for production