[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(..)
Besides from my understanding I think this `if (...) else (...)` code may has concurrency issue, it would be better to change `ProducerCache` into a singleton factory.