State adding

The code for ModelState can be viewed here: https://github.com/django/django/blob/master/django/db/models/base.py#L358

From what I gather, `self._state.adding = True` is the case when you use a proxy model as described in the docs: http://django-parler.readthedocs.org/en/latest/advanced/existing.html

All tests are still green, but this update is not something that I feel 100% on top of :) I can tell, though, that I have both normal translations and proxy model translations running.

Remaining bug: One big problem remaining is that if - say - Portuguese is my current language, i.e. that I'm in `/pt/admin/app/translatable-model/123/?language=de` and a German version does *not* exist. If I add it, it will overwrite the Portuguese version! The German version will also be saved, though.