Respects -Dmapred.job.name when passed in on the command line

This allows users to pass -Dmapred.job.name=<name> on the command line in order to see <name> in the job tracker instead of the class name. It still defaults to the class name if nothing is passed.
will this work? I think it will just change the class name part that you have.

I think it is fine to add, but I am not sure it closes #1044