fix: adding tag w/ name already exists ends in infinite loop (+tests). Thx @miso-belica
Merge pull request #3 from uruz/feature/support-django-1.6

Support both 1.5 and master branches of django
Django commit https://github.com/django/django/commit/3647c0a49a2f4535b8a9aba40e662743e4d53e76 broke taggit. This commit fixes that. Thanks akaariai for the patch (see https://code.djangoproject.com/ticket/19399 for details).