* For items and tasks the owner id is set to admin
* For notification subscriptions, the subscription is deleted
* For comments, I've extracted the user name, email and url and set the guest_name, guest_email and guest_url columns while setting the author_id to identity::guest()->id
Fix for ticket #777.
1) The item_updated event no longer takes the old and new items.
Instead we overload ORM to track the original data and make
that available via the item. This will allow us to move event
publishing down into the API methods which in turn will give us
more stability since we won't require each controller to remember
to do it.
2) ORM class now tracks the original values. It doesn't track
the original relationships (no need for that, yet)
3) Added new events:
item_deleted
group_deleted
user_deleted
