Spark Integration: Streams support for Spark Jobs

Adds support to read Streams in Spark to resolve : CDAP-403

Build Running: https://builds.cask.co/browse/CDAP-DUT102-4

@anew : Please review. Thanks.
Oh yes!! When @jwang47 suggested validating vClass I forgot that it just has to be BytesWritable or Text if the decoderType is not provided and changed it to this. Totally my bad. I will change it now.