This also updates all dependent functionality, including modelform_factory
and modelformset_factory, and the generic views `ModelFormMixin`,
`CreateView` and `UpdateView` which gain a new `fields` attribute.
Credit goes to @SystemParadox. Originally developed at #DjangoCon Europe
but wasn't tested enough to merge in.
For history, please see https://github.com/pydanny/django/pull/4