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.