change BooleanFilter.filter() to set self.is_used as needed
FilterSet.qs() now sets is_used=True on ordering field
FilterMixin.get_filterset_class() now sets self.model if filterset_class was defined
Alters Filterset._meta to allow arbitrary Meta options.
adds filter choices value 'inherit' which makes filterset pull choices from model field
adds DateOffsetFilter and NullBooleanFilter