make it possible to distinguish update and add notifications.

when subscribing to perstore, all notifications are either `'update'` or `'delete'`.  i'm using dstore as a client and it handles `'add'` differently to `'update'`.  it's not possible to infer whether or not an `'update'` from perstore is really an `'update'` or an `'add'`.  it would be good to introduce an explicit `'add'` topic or at least make it possible to infer an `'add'` from an `'update'` topic.