expose 'counters' and 'job_id' to MapReducePipeline output

MapperPipeline returns named outputs:
* counters
* job_id

To expose these from the `ReducePipeline.MapperPipeline` to the
MapReduce pipeline, the outputs must be named and passed through
`MapReducePipeline`
Merge pull request #10 from soundofjw/mr_output_expose

expose 'counters' and 'job_id' to MapReducePipeline output