Add CRON=1 backup setting for quiet backups
Explain why we create a StringIO
Merge branch 'backup-cron-mode' into 'master'

Backup cron mode

If you have your server configured to receive emails containing the output of
cron jobs it is annoying to get long spammy emails from the backup script. This
change adds a 'cron mode' that makes the backup script silent unless something
goes wrong during the backup.

See merge request !1268
More explicit wording of the documentation.
Formatting and sequence of contrubution paragraphs.
Merge branch 'master' of github.com:gitlabhq/gitlabhq