Add Kafka producer

Adds a really basic kafka producer, and update the docs a bit on its usage.

I'm really not a ruby programmer(normally program in scala), so I'm sure this is pretty ugly.  And I should add support for a lot more options with for a kafka producer.  But I thought I'd share this now, if nothing else it can be a starting point for you. Also happy to take comments & feedback, I'll update the pull request.  

my goal is to use this w/ logstash, so I'll also try to add a logstash kafka output module shortly.