[Patch] createrepo --check option

Hi Seth,

as promised, here's my first patch for createrepo to suppress metadata 
recreation, if the timestamps are up to date, including documentation 
changes to createrepo.8 and ChangeLog. 

Works fine here, and I would be very happy to get it applied upstream.

If you have any issues with it, tell me, please.

Thanks,
Pete

P.S.: Could you say a few words to the message right above this one?
      Or to be more concise, can I use different group.xml files on
      different repos and any group definition in them with yum?