Rotate ~/Backup/log/backup.log and add rotated ~/Backup/cron.log

[michiels]

The Backup gem creates ~/Backup/log/backup.log but I don't think it's rotated. Might end up to be a very large file so let's rotate it with log rotate.

Also to catch bugs in cron execution it'd be wise to have our cron jobs output to ~/Backup/cron.log and rotate that too.