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. 
It is very good to have a separate configuration for a different codec of map, but I think you can keep the default value as DefaultCodec, since enabling Snappy appears to be a little too difficult for those who are new to hadoop. You can put your advice in comment, so that advanced users can see that and configure it by themselves.