Add configurable formfields (close #211, ref #163, #187).

This will hopefully at last resolve the problem with nullable CharFields.
Fix recursion problem with fieldset handling in Django 1.6
Fix flake8 error
Don't get too excited with the flake8-patching
Merge pull request #214 from bpeschier/fix-django16-recursion

Fix recursion problem with fieldset handling in Django 1.6
Merge branch 'ultimate-null-formfield-solution'