[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(..)
An executor can run multiple tasks at once though. Do they always have the same config and is the Producer thread-safe. I am not sure how much it is guaranteed that there is one executor per JVM. To me it feels safer to make a Producer per partition. They aren't that heavy to create and there aren't that many partitions relative to data size.