Optionally support jsonb

[andybak]

It should be possible to detect if django-pgjson is installed and if so - use that as the underlying model field: http://djangonauts.github.io/django-pgjson/

This would allow efficient querying and other cool things - especially with Django 1.7 and Postgres 9.4: http://clarkdave.net/2013/06/what-can-you-do-with-postgresql-and-json/