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
If you want to use a Kafka library that compiles against a different Scala version, you have to use the normal artifact (without the `embedded-kafka` classifier), exclude the Scala dependency and include a dependency to the Kafka version you want to use. \[Perhaps this line can be broken? It looks very long. Common with a number of lines.]