[CDAP-505] Added ability to infer IdentityStreamEventDecoder in StreamIn...

Added ability to infer IdentityStreamEventDecoder in StreamputFormat. Updated SparkPageRankProgram to use IdentityStreamEventDecoder.
Should be `vClass.isAssignable(StreamEvent.class)` instead so that user can have either `StreamEvent` or `StreamEventData` as the value type in his MR job.