Apparently we need some getters so that the metadata for the
configuration properties is generated.

fixes #391

With this change the AbstractNotifier isn't specialized on status changes, but
the default behaviour for the existing notifiers isn't changed.
In addition the existing notifiers have been improved so that subclassing has
become easier.
To have existing notifiers acting on other events you need to override
shouldNotify().

fixes #270

The RemindingNotifier sends periodic notifications using a delegate. It must be
added explicitly to the ApplicationContext