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
It's optional since we provide an "embedded-kafka" classifier for the artifact which has the required Kafka classes in it.
For projects that depends on those, they won't need this dependency.
This saves them the effort of installing Kafka 0.7 into their local maven repo.
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