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);
```
> I'm not sure that implementors should be in charge of providing this info. 

They are actually should because this one tell which plugin types to query. So the CTools plugins data provider can query other plugin types (e.g. page manager).