[WIP] Add class column

This PR replaces sonata-project/SonataDoctrineORMAdminBundle#125.

As @rande suggested, I created a new custom column type. You can use as follow:
```php
<?php
$list->add(new ClassFieldDescription(), null, array(
    'classes' => $this->getSubClasses(),
));
```

I am not satisfied with the new `ClassFieldDescription` class. I think some refactoring should be done. So I just submit this PR to show a working POC and start a discussion about how to process and have a cleaner implementation.