Avoid using ```get_all_field_names()``` method

In Django 1.7 (and maybe earlier), this fails since ```._meta.get_all_field_names()``` returns content_type_id (as well as content_type), and then building the form fails since this extra field does not resolve to a model field.
Merge pull request #6 from asfaltboy/django-1.7-support

Avoid using get_all_field_names() method