Make list of denied handlers available via JMX

When a service gets blacklisted by the eventAdmin, there is only a single warn statement in the logs. It would be great, if it would be an error, as a blacklisted service normally results in application problems. And if it's printed as error, it should be catched by any log analyzing mechanism.

On top it would be good, if blacklisted services can be detected by a different way (e.g. by throwing an OSGI event, updating an JMX MBean or something like that). Log parsing is cumbersome if you need to do online monitoring of your application.