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
Long line due to the editor auto-wrapping (hence I didn't notice it). Will fix it.