add decoder options

[talevy]

This PR will allow individuals to apply their own decoder for Kafka messages

example:

# let logstash runtime to load your custom library so you can use it!
$ export CLASSPATH=/path/to/lib.jar

# logstash config
input {
  kafka {
    topic_id => "my_custom_topic"
    decoder_class => "org.logstash.dots.DotsDecoder"
  }
}

[joekiller]

jruby-kafka 1.2.0 is live now so this can continue.

[talevy]

thanks @joekiller!

[suyograo]

LGTM