Added a new ZMQLogbackAppender.

This is a fork of ZMQAppender, but for logback rather than log4j.

This includes some simple changes to LocationInfoData and LoggingEventData
so that they can cope with data from either source.

It's entirely up to you, John, whether you want to pull this.  I don't mind either way.  It obviously introduces a compile-time dependency on logback, and it's not an elegant fork, just a copy-and-paste job.  I thought you might be interested though,