Add feature to clean out old publication tags

I usually turn off CVMFS_AUTO_TAG because it generates so much clutter for replicating. However I think it would be useful to be able to sometimes back up to fairly recent versions, in case of some data accidentally getting deleted. So I would like to see a feature to remove tags older than a certain amount of time. In fact I think it would make sense to have it be the default to remove tags older than a week or two.