Mdbox format for Maildirs

Added mdbox as alternative to Maildir-Formats (only working with dovecot)

Note: Backup-Function differs from Maildir-Backup, because there is no proper way to restore a mdbox-backup INTO an existing Maildir withoud deleting newer mails. Backups will be restored into a subfolder backup-yyyy-mm-dd unter the mailbox. User can then copy his needed mails and in the end deleting backup-subfolder. This includes the possibility of exceeding mail-quota but this must be accepted. It's the users problem if he doesn't delete his backup afterwards. This type of restoring will be included into Maildir-Backups, too - in the next step.