(CDAP-331) Allows more flexibility on the data type for consuming from stream using 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.
Merge pull request #392 from caskdata/feature/enhance-mr-stream-input

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