Make subscribe() and unsubscribe() internals.
Make subscribe() and unsubscribe() internals.

These methods should not ever be called from other
places than the current save() method, hence mark
them as internal.

ctxis/django-newsletter,viaregio/django-newsletter,dsanders11/django-newsletter,ctxis/django-newsletter,ctxis/django-newsletter,dsanders11/django-newsletter,dsanders11/django-newsletter,viaregio/django-newsletter