[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(..)
The idea is to not create a new Producer instance for each partition. Instead, use the instance of a producer already available on that executor. The drawback is this slightly complex caching mechanism - while the advantage is to reduce the overhead of creating a new producer instance and connecting to Kafka etc. 

I am not too familiar with the internals of Kafka - so erring on the side of caution.

You are right in the fact that there is a concurrency issue here - I will fix that in an update later today.