Allow to specify validation group

Hi,

I run into a problem this morning, because of lack of validation group. With this PR, your controllers now can implement the new ValidationGroupProviderInterface (ideally, input filters should support named validation groups out of the box, this is something I'll think about):

```php
class PostController extends AbstractRestfulController implements ValidationGroupProviderInterface
{
    public function getValidationGroups()
    {
        return [
            'post' => ['username', 'password'],
            'put' => ['username']
        ]
    }
}
```

The data validation trait will automatically detect if the controller implement this interface, and get the appropriate input filter.

Thoughts? ping @danizord