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
For example, to create a dependency on `kafka_2.10-0.7.2` (a version of `0.7.2` that compiles against `scala-2.10`):