Fixed problem with auth.user in the south migrations. Further continuati...

...on of e506c0941bfed1104394ffc176484c928685080f.

Hey, 

I've fixed an issue with the south migrations when you don't use the auth.user model, but a custom model defined in settings.AUTH_USER_MODEL. This should fix issues like  
```
ValueError: Cannot successfully create field 'user' for model 'articlerevision': "The model 'user' from the app 'auth' is not available in this migration.".
```