Added possibility to append kafka keys to events

This is related to: https://github.com/joekiller/logstash-kafka/issues/38 and pull request: https://github.com/joekiller/jruby-kafka/pull/12

By the way. I had problems with access to kafka fields (decorated events). They weren't accessible in ie. grok and elasticsearch output had problem with serializing them into json. This seems to be similar to problem with twitter input: https://github.com/elasticsearch/logstash/issues/1471. To make it work i had to apply this commit: https://github.com/jpodeszwik/logstash-kafka/commit/1e71b39b49fc3fd3d47bd1eea966e41005f3b978 (not included in this pull request). Should i also include it here?