Add a SimpleKafka output

This is a simple output for Apache Kafka. It uses the [poseidon](https://github.com/bpot/poseidon) gem, which despite its README warnings about not being production ready has served us well for several months now.

I see there's work happening in https://github.com/elasticsearch/logstash/issues/1472 that is probably a more complete solution, but I'll put this up for consideration anyway. Perhaps someone will find it useful.