[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(..)
I do, but it is a completely different design of doing this. I'm working on something that allows writing to kafka from DStream and RDD but i'm headed to talk at a conference and won't be able to get back to this code until next week. Getting this functionality in asap is important :) so I'm glad you are doing this.
I might suggest 'writeToKafka' be changed to 'saveToKafka' for api naming convention reasons.