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
I re-phased as:

```
It's optional since we provide an artifact jar that has the required Kafka classes in it, using the
"embedded-kafka" classifier. Hence, for projects that depends on the "embedded-kafka" artifact,
they won't need this dependency.
Making this optional will prevent maven to treat this as a transitive dependency, hence the user doesn't need install Kafka 0.7 into local maven repo manually (as kafka-0.7.2 is not in maven central).
```