Not possible to use with django.contrib.auth.models.User

I noticed that the User models were never getting cached, so try to override the default manager by monkey patching it

```
User.objects = CacheBotManager()
```

This results in an error though, because queryset.model is None. Is there another way that I should be overriding the User manager, or is there a fundamental limitation with cachebot that means I can't cache User objects?
