Fix VATINField in django.contrib.admin

When I add a VATINField to my model and try to edit it with django-admin, I get this exception:

```__init__() got multiple values for keyword argument 'widget'```

This is caused by the admin inserting `django.contrib.admin.widgets.AdminTextInputWidget` in kwargs as widget.

This PR puts the VATINWidget in kwargs, thus overriding the admins' widget argument.