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. This should be automatically derived from the folder this file is in and `hook_ctools_plugin_directory`. We can probably add this `module` and `type` in the process function.