Allow using custom primary keys on models.

This issue is related to https://github.com/bfirsh/django-ordered-model/issues/29.

Basically, I'm using https://github.com/dcramer/django-uuidfield as primary keys for my models and OrderedModel wouldn't set the order properly for those models. The reason for that is that OrderedModel will set the order only when the saved model has no id set and UUIDField sets the id before saving the model.

Instead of depending on model id, the code will now check if the order field isn't set.