Change to delete old backups by stating how many you want to keep.

This change is intended to be used in a cron script that runs daily to prune your basebackups in s3.  You specify how many basebackups you want to keep and it deletes all the basebackups and wals that are older than that.  So if you have 10 basebackups and you run "wal-e delete keepcount 3" it would delete the 7 oldest basebackups.