Add a SymfonyCache Adapter

I've added a `SymfonyCache` adapter to clear Symfony cache (or only a subdirectory of it, for instance translations or twig) on multiple servers.

Here is the configuration:

```yml
sonata_cache:
    caches:
        symfony:
            token: s3cur3 # token used to clear the related cache
            servers:
                - { domain: kooqit.front1.local, ip: 127.0.0.1, port: 80}
                - { domain: kooqit.front2.local, ip: 127.0.0.1, port: 80}
```

Now, imagine you want to clear Symfony translations cache on all servers, just call:

```php
$this->get('sonata.cache.symfony')->flush(array('translations'));
```

This is done as the same way as `ApcCache` with a socket connection making an HTTP call on a specific route secured by a token and use the Symfony `Filesystem` to remove the directory.

I'm open to comments if any //cc @mremi