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
- Ignores changes in ordering (done via update, so `save()` signals are not generated)
- Restores the old value when loading from a `history_object`, which may
  result in non-deterministic ordering until `set_RELATED_order()` is
  called again.

The ordering could be 'frozen' by calling `set_RELATED_order(get_RELATED_order())`, but it's not clear that would be better, especially considering multiple restores from history.

See #140 for the full discussion, and PR #141 for an aborted attempt to drop the ordering field in the history model.