StateField with abstract models

Changes submitted in this pull request allow adding a StateField to an abstract model.
It behaves as expected: ``_state_log_model`` and other model methods are added only to concrete models.
Also couple minor changes that allow using test_proj to run tests:
* ``get_state_info().initial`` is referenced [here] (https://github.com/vikingco/django-states2/blob/master/django_states/models.py#L119), but not implemented
* ``test_proj/settings.py`` uses ``os`` but doesn't import it

And it also implements suggestion #56