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.
I'd leave this as a regular defn, in case users want to do their own post calls. We've seen this with the email stream, where it's convenient for users to leave the stream API temporarily.