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
The library supports both Kafka 0.7.x and 0.8.x. Since artifacts for Kafka 0.7.x are not available in the Maven
central repository, the project includes an artifact jar for Kafka 0.7.x. You'll need to install the artifact into your local
maven repository before building the library: