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.
Yes. Here is what I added: 

  @Override
  public <T> T readFromStream(String streamName, Class<?> vClass) {
    return readFromStream(streamName, vClass, 0, System.currentTimeMillis(), null);
  }