UnicodeSlugField for Django

I thought others might encounter the same problem as I did, so I'll share my solution and possibly get some feedback if there is a better way to do it.

Problem: If you use slugs generated by unicode-slugify with Django's SlugField, you will be prevented from saving a form with a SlugField (e.g. in the Django Admin) if the field value is invalid by [Django's slug standard](https://github.com/django/django/blob/master/django/core/validators.py#L187).

Solution: Replace SlugField with a new UnicodeSlugField which inherits everything from SlugField except that pesky validation. Add a new validator using unicode-slugify.