Update urls.py to Django 1.6+; fix issue #15

This is my first pull request, so I hope it's done properly. I updated urls.py to [work with Django 1.6+](https://docs.djangoproject.com/en/dev/internals/deprecation/#deprecation-removed-in-1-6) and updated cart_modifier.py to fix issue #15 following [django-shop's cart_modifiers_base.py](https://github.com/divio/django-shop/blob/master/shop/cart/cart_modifiers_base.py).