Improvements for ordering in custom exports

http://manage.dimagi.com/default.asp?86347#695228
@dannyroberts would you take a look at this and verify that `self.default_order` no longer needs to be used for sorting? If not, I suggest that all `default_order` code should be removed from this file and also from [custom_export_helpers.py](/lwyszomi/commcare-hq/blob/86347-2/corehq/apps/export/custom_export_helpers.py) and possibly [models.py](/lwyszomi/commcare-hq/blob/86347-2/corehq/apps/reports/models.py) (`get_default_order`) since it appears to be unused.