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-DUT102-8

@anew : Please review. Thanks.
The differences is that the "set" method takes the class name, and the "get" method returns a `Class` object. Maybe it's better to call this method `setDecoderClassName`.