Added mechanism to treat kafka message as JSON and to turn off per message logging

* Added Configuraiton option and implementation to the
  ElasticsearchProducer to treat the Kafka message as JSON and not a
  String. Updated the README as well.
    * The default is the old behavior (treat the kafka messages as a string)
* Added Configuration option and implementation to the KafkaWorker to
  not emit a log entry for each kafka message (that would overwhelm the
  logs with real big data)
    * The default is the old behavior (emit a log entry per message)