Added support for sending messages to Graylog2

Uses the GELF format and requires at least version 1.4.0 of the GELF gem as logger for proper formatting. (https://rubygems.org/gems/gelf)

No dependencies added to lograge. It just returns a Hash, picked up by the GELF gem logger.

The `log_output` in the tests is always a String, no matter what `process_action` actually returned. `process_action_graylog2` is returning a Hash, not a String. I did not want to change so much in the tests and just test against the String representation of the Hash now.  (Which is fine IMO)

![Screen Shot 2013-01-26 at 19 43 56](https://f.cloud.github.com/assets/35022/100082/601b0078-67e8-11e2-9991-ec225b962823.png)