Support for deleted items

Hi again,

These commits add the ability to keep track of deleted objects. I think I've seen an issue about that but cannot recall which # it was.

The behaviour is controlled by the USE_DELMODEL setting. If False (default), nothing changes compared to the current behaviour (i.e the action is deleted with the object). If True, a DeletedModel django model is populated with representations of the deleted objects. It's good if one wants to keep records of deletions in the activity stream, such as 'The object "X" has been deleted'.

Have been using it for a few weeks now (not in production yet though) without any problems.
Feel free to comment / make suggestions.

Thomas