[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(..)
AFAIK, each executor runs a single JVM per application. The idea is to just reuse the Producer in the JVM.

I maybe wrong though (at least it seems that way in YARN mode at least - the executors are allocated when the application starts, and I don't think they are shared between applications).

If that is not the case, I think we'd have to just create a new Producer per partition - which is a higher overhead though.