Allow foreign key attributes in list_display with '__'

This is based on DjangoSnippet 2996 which was made by Kpacn.

1. This library is on PyPI so you can install it with:

   pip install django-related-admin
   

or from github:

pip install git+https://github.com/PetrDlouhy/django-related-admin#egg=django-related-admin

2. Add "related_admin" to your INSTALLED_APPS setting like this:

   INSTALLED_APPS = (
       ...
       'related_admin',
   )
   

Just use it instead of model.Admin:

from related_admin import RelatedFieldAdmin
from related_admin import getter_for_related_field

class FooAdmin(RelatedFieldAdmin):
    # these fields will work automatically (and boolean fields will display an icon):
    list_display = ('address__phone','address__country__country_code','address__foo')

    # ... but you can also define them manually if you need to override short_description or boolean parameter:
    address__foo = getter_for_related_field('address__foo', short_description='Custom Name', boolean=True)