enable production logging into json file

If the property 'productionLogging' is non-null, a Logback appender will be added that writes every log message into a json file. Would ideally be configurable via logback.xml file, but unfortunately logback tries to instantiate the class by name, which doesn't work with contrib bundles.

Still missing:
 * more configuration options (waiting for configuration solution)
 * log message as json (currently, the message is added as message: "[msg.toString()]"