Initial implementation of Time Machine style backup pruning.

Within 24 hours, all backups are kept. Within 1 month, only the most
recent backup for each day is kept. For all previous backups, only the
most recent of each month is kept.

This is not *quite* the same as Time Machine, but this implementation
is a lot easier to do since it is based on string comparisons of the
dates and doesn't require any "hard" date logic.

Also this commit just 'echo's what will be deleted, and does not
actually delete anything yet because I am still testing it.