Django 1.8 compatibility

Fixed most of the issues by removing the shared instance assignments to the translated instances. Apparently this didn't break anything (test are running again on 1.8 master). Not sure adding the exclude = [] to translationformset_factory makes sense. I added this to silence the "ImproperlyConfigured: Calling modelformset_factory without defining 'fields' or 'exclude' explicitly is prohibited" exceptions.
Also, get_deleted_objects() of contrib.admin.utils seems to have changed and returns an extra "model_count" param which leads to a "too many values to unpack". Added some compatibility code to handle that as well. There is still one test failing regarding language fallbacks. Maybe somebody could check that out. I verified that this still runs on latest stable (1.7.1). All tests pass there.