First complete version of Kafka flowlet

Quick links:

README: https://github.com/caskdata/cdap-packs/blob/feature/kafka/cdap-kafka-pack/cdap-kafka-flow/README.md

BUILD: https://github.com/caskdata/cdap-packs/blob/feature/kafka/cdap-kafka-pack/cdap-kafka-flow/BUILD.md
This library contains classes for consuming data from Apache Kafka. It supports Kafka versions 0.7.x and 0.8.x. If you are unfamiliar with Apache Kafka, more information is available at the [Apache Kafka Project Site.](http://kafka.apache.org)