Graylog2 Extended Format Decoder

A simple decoder for the Graylog2 Extended Format for logging based on some code I saw in a blog @trink had assisted with. This should work with the specification for GELF. The logical next step would be a Graylog Output for the server but it is sufficient for my uses to simply ingest GELF so it can be tossed at ES+Kibana.
I realize the doc generation really spews a ton of warnings but we can at least eliminate the graylog ones. I created a bug for the others #1099

/work/git/heka/build/heka/src/github.com/mozilla-services/heka/docs/source/sandbox/decoder.rst:17: WARNING: Title underline too short.
../sandbox/lua/decoders/graylog_extended.lua:17: WARNING: Definition list ends without a blank line; unexpected unindent.