Add backup-each-save

Backup-each-save is a heavy-handed approach to having backups of every file modification. It's discussed on the wiki: http://www.emacswiki.org/emacs/BackupEachSave and I learnt of it when John Wiegley mentioned it in his talk at emacsconf.

Basically, it's an alternative to git-wip. I'd love to see it on MELPA.