Added ZMQLogbackAppender.

This is a fork of ZMQAppender but for logback instead of log4j.

This includes an additional constructor on LoggingEventData, so that data
from either framework can be handled, and a dependency on logback.
Fix NPE by setting some location info.
Put tags=[] instead of tags=null on the wire.  Logstash fails to parse
the former.
Put the formatted message into the JSON to send to logstash, not the
one with the placeholders.
Switch from jeromq (pure Java) to jzmq (native code).

jeromq seems to hit a JDK bug with epoll.  This has been worked around
by Netty (see EPOLL_BUG_WORKAROUND in that code base) so there's a known
solution.  Rather than port that over to jeromq though, I've decided to
try the jzmq bindings on the assumption that the native code will be
better tested.