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

Added ability to infer IdentityStreamEventDecoder in StreamputFormat. Updated SparkPageRankProgram to use IdentityStreamEventDecoder.
I thought about this. Correct me if I'm wrong, but there is only one test that uses `TestBatchStreamIntegrationApp` - `TestBatchStreamIntegration#testStreamBatch`. The other test uses `NoMapperApp`. Should I add another test with a different mapper? This way I'll also need -
# another `MapReduce` that extends `StreamTestBatch` and overrides the mapper
# add the new `MapReduce` in `TestBatchStreamIntegrationApp#configure()`. Start it only in the new test.

The other way could be that I can keep the mapper input value as `BytesWritable`, instead change the reducer input value in `NoMapperApp` from `Text` to `StreamEvent`. That way all three input values can be covered without an extra test.