Add plugin for output to Keen IO

Like it says on the tin! :smile: 

We're clojure noobs and this was largely snagged from the Pagerduty and OpenTSDB plugins.

We like to batch events for performance reasons but I didn't see any examples of outputs that batch things up. I know this can be accomplished inside riemann but it will still cause us to send multiple POSTs instead of one big one periodically. Thoughts?

Thanks for any tips, improvements or suggestions.
May want to indent this to align with final-event; it looks like a part of the let body right now.