Since the Notifiers shouldn't throw exceptions, we let all extend the
AbstractEventNotifier which gets us a enabled property and catching
exceptions which may occur when notifiying.

fixes #382

To easily add new mvc controllers (e.g. from extensions) a new
annotation `@AdminController` is added.

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

fixes #202

This adds a new FilteringNotifier which can use filters to suppres certain
events from being notified of. Two filter implementation are provided, one
filtering by name and one by id. Additional when a FilteringNotifier bean is
present, the autoconfiguration creates a REST-controller to add and remove
filters at runtime.

added CompositeNotifier to allow for multiple notifiers. Only used when multiple notifiers are configured and no primary notifier is already configured.

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

 * Use @EventListener instead of ApplicationListener-interface
 * Don't inherit ClientApplicationEvent from ApplicationEvent - so JournaledEvent isn't needed anymore
 * added eclipse formatter rules
 * reformatted all files

The mail notification is enabled if a mailSender is present specified via spring.mail.* properties.
On default the mail is sent to root@localhost and the chages from UNKNOWN to UP are ignored.