[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(..)
If there is a way to keep the Producer around, that is not terribly complex, it would be awesome. As far as I could see, the ProducerCache thing in an earlier version was what I could think of. 

I think we could extend this patch to also write an RDD to Kafka (it can use the same semantics). Do you have a repo which I can peek at?