enhancements to kafka-websocket

i've been a busy little bee this week so kafka-websocket now supports keyed
messages and user-defined transforms for messages. the latter is pretty
raw, improvement suggestions in the form of pull requests welcome!

https://github.com/b/kafka-websocket/commit/6430fd522c0e0305995f648511726da0b616d2eb

https://github.com/b/kafka-websocket/commit/4d5f4feb48b5bb3efa4a48a92a276b6274917bc1