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.
Is this necessary? The call to the `setStreamInputDataset` will deal with it (by calling `StreamInputFormat.setStreamEventDecoder`). Also, the current logic is wrong in a way that if user provide decoderType, the vClass doesn't need to be byte[] or text.