KafkaSink for metrics2

The KafkaSink, acting as a KafkaProducer, publishes metrics to a Kafka topic.
The regular kafka configuration parameters can be directly specified in the hadoop-metrics2.properties file under the metrics2 sink prefix (ex. test.sink.kafka.bootstrap.servers or client.sink.kafka.compression.type).
Addition of the KafkaSink will enrich the metrics2 sink repertory that already includes sinks for Graphite and Ganglia.