[CDAP-331] Allows more flexibility on the data type for consuming from stream for MapReduce

- Added auto-detection based on Mapper/Reducer input type to support Text or BytesWritable to carry stream event body
- Added support for user to provide a custom StreamEventDecoder to transform StreamEvent into Key/Value pair for MR input.
What I meant was, it makes sense to mention in this comment why it is assumed to be LongWritable.