[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(..)
@helena - Please see discussion earlier on this thread. For now, we can do this for simplicity, but an earlier commit does cache the Producer. If we see this as too inefficient once we run it and test it out on real use-cases, we can bring back the cached Producer code.