Allow to not register the ModelAdmin for Profile and User

This is needed because if another app is also registering these models it would cause problems.

If the other app is before `userena` on the app list, `userena` will override the `Use`r admin and raise an `AlreadyRegistered` for the `Profile` admin.
If the other app is after `userena`, we could unregister the `User` and `Profile` admins, but then we could not override the templates.

BTW, This is already mentioned in (closed) [issue 34](https://github.com/bread-and-pepper/django-userena/issues/34), but I think it deserved a patch.