Adds support for revision object that has a Document based user

Until now the `Revision` model only supported an associated `user` that was a Django ORM model from `models.Model`, so this PR adds support for an associated `user` that is based on the mongoengine `Document` class.

Deals with issue https://github.com/snormore/django-mongoreversion/issues/1

@jnormore for review please.