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. 
For two reasons: (i) Since a vendor wouldn't look deep into the compression options, we want to create compression options that yield good performance. Turning off map.output.compress is never a good idea (ii) we want things to be consistent across MR1 and MR2. For MR1, we are not turning off map.output.compress (which is the right thing to do).