Versioning

Our versioning module is build on top of django-reversion. Reversion handles all the heavy lifting of actually creating the revisions. What it does not do by default is version comparison of the type that's present throughout PyRIGS. So we have custom logic for this in versioning/versioning.py.

This compares model fields, and sometimes related models, like payments for invoices: it recurses one level to do this.