Allow subclass overriding of Validator

I want to be able to use Laravel's Validator's sometimes() feature with Ardent:

```
$v = Validator::make(...);
$v->sometimes('reason', 'required|max:500', function($input)
{
    return $input->games >= 100;
});
```

Allowing subclass overriding of Ardent::makeValidator() seems the easiest way.