Fix #140 - Convert OrderWrt to IntegerField

An OrderWrt field named _order is added when the
Meta.order_with_respect_to option is used on the model.  This code:
- Switches it to an IntegerField in the historical model
- Restores the old value when loading from a history_object, which may
  result in non-deterministic ordering until set_RELATED_order() is
  called again.
Merge pull request #142 from jwhitlock/order_wrt_included

Fix #140 - Convert OrderWrt to IntegerField