[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(..)
Hi Hari, a simple question, will `Producer` be shared between each partition? IIUC your code seems reuse Producer when it is already created, is there any specific purpose to do so?