"select_related" shouldn't raise NotImplementedError

Currently, calling ``select_related`` with more than one level raises a ``NotImplementedError`` (see https://github.com/KristianOellegaard/django-hvad/blob/master/hvad/manager.py#L439). This is inconsistent with Django's behaviour (which just ignores relations it cannot satisfy). Also. just ignoring instead of noisily complaining would allow me to already go and set up my queries for the case that django-hvad will support deeper relations one day.