Compression-related changes to HiBench YARN branch

We noticed performance discrepancy between Master and YARN branch. After lot of experimentation and analysis, we found that the primary problem was compression options. Contributing some of the changes back upstream. For example:  snappyCodec should be used for map.output.compress instead of defaultCodec; by default compression should be turned off, etc. 
Since HiBench is still in its infancy and nobody has officially published against it, it would be best to make changes without the fear of regression, otherwise improving HiBench becomes very hard. Besides, in the "kit" model for a benchmark, a vendor is expected to run the benchmark without any modifications. There is very significant performance difference for at least three benchmarks with compression turned on/off. I would suggest keeping the default as off, since most vendors wouldn't know that turning it off can give a significant performance boost.