User model fields accessors clashes issue solved

Running the provided Django example was impossible with Django 1.6, it gave the following error:

```
python manage.py syncdb
CommandError: One or more models did not validate:
app.customuser: Accessor for m2m field 'groups' clashes with related m2m field 'Group.user_set'. Add a related_name argument to the definition for 'groups'.
app.customuser: Accessor for m2m field 'user_permissions' clashes with related m2m field 'Permission.user_set'. Add a related_name argument to the definition for 'user_permissions'.
auth.user: Accessor for m2m field 'groups' clashes with related m2m field 'Group.user_set'. Add a related_name argument to the definition for 'groups'.
auth.user: Accessor for m2m field 'user_permissions' clashes with related m2m field 'Permission.user_set'. Add a related_name argument to the definition for 'user_permissions'.
```