CIRCLE / django-taggit

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

Prevented infinite loops on Tag.save()

remember to set attname if django < 1.7

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

Use Django's atomic when available.

Update api.txt

Minor fix, 'slugs' is a method bound to TaggableManager

Get prefetch queryset

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

This reverts commit 188182b9.

Added support for prefetch_related on tag fields

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 :/).