Model versioning

- Adds the ModelVersion model to versionate the objects

- migration: 

```
  psql -d mootiro_maps < scripts/migrations/2013-11-20_add_model_versioning/db_migration.sql
```

- Add the async task `model_versioning.task.versionate(user, object)` for versioning.

- Has no reversion yet, works like a passive logger

- Completely remove django_reversion