[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(..)
My understanding is that the tasks are all part of the same application - so it should be fine (we are not sharing data between applications).

The Producer is thread-safe (I tested that while writing a KafkaChannel for Flume), so that should be fine.

But if it is not that heavy to create per partition (it would be around once per 2 seconds or so), then I will go ahead and do that. That does have the benefit of avoiding some of the complex code meant to handle reusing the producer.