Implements ConfigurationInterface to allow usage of config:dump-reference

This PR implements Symfony\Component\Config\Definition\ConfigurationInterface for the  bundles configuration.

This allows the usage of 

```./app/console config:dump-reference fos_comment```

As of now the result is:

```php
[LogicException]
Configuration class "FOS\CommentBundle\DependencyInjection\Configuration" should implement ConfigurationInterface in order to be dumpable
```