Some related objects that should not be deleted

[psychok7]

This will hopefully fix https://github.com/pinax/pinax-models/issues/13 regarding some related objects that should not be deleted.

Some more testing feedback would be appreciated to ensure this works properly ;)

[psychok7]

This should actually be activated by a setting mimiquing the behaviour of the on_delete Cascade and/or Set_Null https://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.models.ForeignKey.on_delete

Django Default is Cascade which is the current behaviour without this PR