admin.py needs a couple of fixes

[gtoffoli]

I'm using this useful app with Django 1.6 and will try it with 1.8
Module admin.py needs 2 small fixes:

1. Change the definition of UserWithMPTTChangeForm by replacing Group.tree with Group.objects

   class UserWithMPTTChangeForm(UserChangeForm):
   groups = TreeNodeMultipleChoiceField(queryset=Group.objects.all())

2. Import models.py, even if no global object is being referred directly; otherways the admin autodiscover function could read the definition of class UserWithMPTTChangeForm before the Group model has been extended with contribute_to_class; in this case I got the following error message:

   AttributeError: type object 'Group' has no attribute '_mptt_meta'