Added the ability to save the job's comment field to the Accounting Storage db

 - Renamed the Accounting Storage database's "DerivedExitString" job field to
    "Comment".  Provided backward compatible support for "DerivedExitString" in
    the sacctmgr tool.
 - Added the ability to save the job's comment string to the Accounting
    Storage db (to the formerly named, "DerivedExitString" job field).  This
    behavior is enabled by a new slurm.conf parameter: AccountingStoreJobComment
 - Updated tests 7.13, 7.14 and the html pages to reflect renaming of the
    DerivedExitString to Comment