Merge branch 'master' of /home/git/repositories/florian030/ispconfig3

install/sql/incremental/upd_0060.sql

install/sql/ispconfig3.sql

interface/lib/classes/plugin_backuplist_mail.inc.php

interface/web/mail/form/mail_user.tform.php

interface/web/mail/lib/lang/en_mail_backup_list.lng

interface/web/mail/lib/lang/en_mail_user.lng

interface/web/mail/mail_user_edit.php

interface/web/mail/templates/mail_user_backup.htm

interface/web/mail/templates/mail_user_backup_list.htm

server/lib/classes/cron.d/500-backup.inc.php

server/lib/classes/cron.d/500-backup_mail.inc.php

server/plugins-available/backup_plugin.inc.php