CIRCLE / django-taggit

Fixes gh-219.

This accepts "to" as kwarg, and provides it via `deconstruct`, and also
changes `deconstruct` to always provide/set "through".

This is more explicit and since the manager always uses a through model
internally, this describes it better.

According to the documentation [1], non-default kwargs need to get
returned.

1: https://docs.djangoproject.com/en/dev/howto/custom-model-fields/#field-deconstruction

Note: Now Tag.save() can raise an IntegrityError in case of concurrency
issues, your app will need to handle that.

Proper (hopefully) fix for https://github.com/alex/django-taggit/commit/20a8063c4deb8fb3c5440787cd8114cb2fdf8537#commitcomment-4715554

This reverts commit 188182b9.

This is pretty barebone for now, but should work till a nice solution is found
(Model validation in Django is stricter than it needs to be :/).

Doesn't seem to be needed now, but better be safe than sorry.