Allow NotificationType objects to be registered dynamically

If you look at http://developer.atlassian.com/jira/browse/JTOOL-20 you'll see where some code has been added to create a custom notification type. Unfortunately you also have to modify notification-event-types.xml in order for it to show up on the Notification Type management screen. It would be useful if plugins could define their notification types and register or have them discovered so that they could be added to this list.