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.
```
Graylog2 Extended Logging Format Decoder
-------------------------
/work/git/heka/build/heka/src/github.com/mozilla-services/heka/docs/source/sandbox/decoder.rst:17: WARNING: Title underline too short.

Graylog2 Extended Logging Format Decoder
-------------------------
/work/git/heka/build/heka/src/github.com/mozilla-services/heka/docs/source/sandbox/decoder.rst:18: SEVERE: Problems with "include" directive path:
InputError: [Errno 2] No such file or directory: '../sandbox/lua/decoders/graylog_decoder.lua'.
```
You probably want to add an item to decoders/index.rst and index_noref.rst too.