Allow redirects in custom actions

With the new "Pages", it is easier to create customized panels to deal with modifying data in pivot tables, as well as other custom, free-form functionality. Because Administrator doesn't currently have the ability to display arbitrary links, it seemed logical to use the existing Custom Actions handler.

Since we're already checking for a Download type, it was easy to add support for redirects. The model configuration would look something like:

```php
    'actions'     => array(
        'add_badges' => array(
            'title'  => 'Add/Edit Badges',
            'action' => function (&$model) {
                return Redirect::route(
                    'admin_page',
                    array(
                        'page'    => 'admin.userbadges',
                        'user_id' => $model->id
                    )
                );
            }
        )
    ),
```

Any method that returns type \Illuminate\Http\RedirectResponse is permitted.