Notification rewriting causes notifications to be missed

[eythian]

It seems that the persistent notification is just rewritten when a new event comes in, rather than a new one created.

If a new one were created, then notifications would automatically propagate to other things that monitor for new notifications, such as the Pebble smartwatch app. This would mean pebble support (as talked about in #258) would come for free. At the moment, it doesn't trigger as a new notification would, and so notifications get missed.