Support for django's datetimes queryset method

Hello,

As of Django 1.6, django introduced a new method on the```Queryset``` class called ```datetimes``` which does special filtering for```DateTimeField``` objects.

You can read more about it [here](https://docs.djangoproject.com/en/1.6/releases/1.6/#time-zone-aware-aggregation).

The problem is that now, when referring to datetime fields on the master model, django will throw a```FieldDoesNotExist``` error.

Currently hvad overrides most if not all```Queryset``` methods to handle the field magic with both master and child models, so I believe this is all that's missing.