Merge branch 'release/2.5'
SimpleDataset: include non-model queryset fields

This changes SimpleDataset to use the same list of field names as a
ValuesQuerySet would include, which will include calculated fields added
using aggregates or `select_extra`
Merge branch 'export-non-model-fields' of git://github.com/acdha/django-tablib into develop
Upped version.
Merge branch 'release/2.6' into develop