FLUME-2498: Implement Taildir Source.

FLUME-2500: Add a channel that uses Kafka.

FLUME-2523: Document Kafka channel.

FLUME-2560: Add Kafka Channel to dist tarballs.

FLUME-2562: Add metrics for Kafka Source, Kafka Sink and Kafka Channel.

FLUME-2729: Allow pollableSource backoff times to be configurable.

FLUME-2734: Kafka Channel timeout property is overridden by default value.

FLUME-2781: Kafka Channel with parseAsFLUMEEvent=true should write data as is, not as FLUME events.

FLUME-2801: Performance improvement on TailDir source.

FLUME-2823: FLUME-Kafka-Channel with new APIs.

FLUME-2901: Document Kerberos setup for Kafka channel.

FLUME-2947: Upgrade Hive and thrift dependencies.