Tombstone events instead of deleting them

[osteele]

Instead of deleting events, tombstone them. This allows for easier audit and recovery after accidental deletion.

To implement:

• Add deleted_at = DateTimeField() to the Event(Document) definition.
• GET /events queries on (untested) deleted_at: None.
• DELETE /events/{id} sets event.deleted_at to the current timestamp.

The use of a timestamp allows us to garbage collect (truly delete) tombstoned events at a later date, although this issue calls for neither a cookbook recipe nor code to do this.