Support logging via GELF, for sending to Logstash

Uses a Log4j appender to deliver log events as compressed JSON objects in
Greylog Extended Log Format via UDP, TCP, or Redis. Disabled by default.

Requires in classpath (such as /usr/lib/hadoop/lib/):
* json_simple-1.1.jar (package: libjson-simple-java)
* logstash-gelf.jar (https://github.com/mp911de/logstash-gelf)

Based on these instructions: http://www.paluch.biz/blog/95

Change-Id: I39c2a41c2d776e76a43fc921603dbb5b66096ebf