RelatedNullFilterSpec: django-admin custom filter all/null/not null/choices
A simple django-admin filter to replace standar RelatedFilterSpec with one with the "All"/"Null"/"Not null" options. It applies to all relational fields (ForeignKey, ManyToManyField). You can put the code in the `__init__.py` or wherever you want. The `_register_front` idea is copied on [this snippet](http://djangosnippets.org/snippets/1963/)
- filter
- admin
- null
- filterspec
- ManyToManyField
- ForeignKey
- not-null