[SPARK-4122][STREAMING] Add a library that can write data back to Kafka ...

...from Spark Streaming.

This adds a library that can writes dstreams to Kafka. An implicit also has been added so users
can call dstream.writeToKafka(..)
The broadcast is only for the configuration -- to avoid shipping that with each task in the closure (since that is reused). Do you see an alternate way of doing it?