Add auth & sites models that are registered by default

Adds an `admin2.py` file to the main `djadmin` app. This registers a `ModelAdmin2` for `auth.User`, `auth.Group` and `sites.Site`.

Includes tests that check these are indeed registered.

Fixes #22 and #250

Worth noting in Django 1.6, [the sites framework will no longer be admin registered by default](https://docs.djangoproject.com/en/dev/releases/1.6/#simplified-default-project-and-app-templates).