WIP: Discovery resource

#198

Apart of adding test, I still need to figure out how to do sorting on multiple values. But the filtering works, so you can already get all resource with ``minor_version > 4``

```php
  $handler = restful_get_restful_handler('discovery');
  $request = array(
    'sort' => '-name',
    'filter' => array(
      'minor_version' => array(
        'value' => '4',
        'operator' => '>=',
      ),
    ),
  );
  $result = $handler->get('', $request);
```
This is probably generic enough to be a good candidate for a more general context.