User CRUD

Ref: #2
yes :s

But we have an issue for now with extending the `PermissionsMixin`: given that the mixin has M2M with fixed related_names, it creates a clash as soon as more than one model inherit from it.

I've discussed with @xordoquy about this, and there is workaround (i.e. dynamically only making non abstract the user model really used), but I wanted a bit more time to think about it, and be sure we need to pull in the mixin for our rights management.

Back to `is_superuser` itself, do you think we should have the field on our abstract base model in the meantime?