Fix CustomInlineFormSet to allow customization

This changes does allows to not override the form generated by the admin and just patches the `default_order_field` field. This permits further customization in the modeladmin (e.g.: by using `formfield_for_dbfield` method) and allows better integration with other django applicatons (e.g.: django-admin-enhancer)
Merge pull request #9 from yakky/master

Fix CustomInlineFormSet to allow customization