Fix TranslatableAdmin get_object() to not use fallbacks.

The fix for [Nani issue 41](https://github.com/ojii/django-nani/issues/41)
results in TranslatableAdmin returning an object with a different translation,
instead of no translation, when editing an object in the admin. Applying
changes made on a form can then apply them to the wrong translation.

If this change is omitted, then the real fix for TranslatableModelForm
breaks the admin and this is caught by the tests.
Use assertEqual in tests instead of assertTrue.

Makes test failure messages more useful. No practical change except that
one wrong assertion is fixed:

	self.assertTrue(response.status_code, 200)
Fix #142: allow access to translated attributes during model save.

Change the way that TranslatableModelForm.save() constructs the instance, to
ensure that translations are created and populated before we try to save the
master instance.