updated for jruby-kafka 0.2.1, style and documentation

  * updated Kafka output sendMsg to send_msg
  * style fixes
  * fix Kafka output test to use send_msg

Created per request: https://github.com/elasticsearch/logstash/pull/1764

The fix from https://github.com/elasticsearch/logstash/issues/1738 seemed to slip by this output so I added the fix to this PR as well.
  * Removed topic_id from producer options as it is unnecessary and created a bug with later jruby-kafka.

@timestamp needs to be a string key as well.