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

Tests in Abstract*.class are not executed during the maven build, so I moved
the tests for code in AbstractNotifierTest to MailNotifierTest

 * 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.