Get rid of django-fusionbox

[Django-fusionbox](https://github.com/fusionbox/django-fusionbox) is not really reusable. It's an unorganized collections of miscellaneous helpers.

This patch get rid of django-fusionbox dependency, it uses [django-argonauts](https://github.com/fusionbox/django-argonauts) instead for JSON Rest views. And it copies the small features (like PhoneNumberFields, QuerySetManager) into widgy.

It also backports `QuerySet.as_manager()` into widgy.