Update.rst partially rewritten

- Removed backup specific procedures
- Default updating procedure now uses internal application 'Updater'
- Some commands added
Commands changed to backup.rst

backup.rst improved with code coming from previous version of update.rst. Every backup now as a date. rsync preserves ACLs, extended attributes (like SELinux attributes).
restore.rst added

Commands to restore folders and database are now available.
restore.rst added to index.rst
index.rst page order is changed
Update migrating.rst

Updating migrating.rst according with new maintenance structure.
Non-interactive password for MySQL/PostgreSQL commands
Fixed MySQL commands: unneeded space removed
Merge pull request #186 from frafra/stable6

Improved backup/restore documentation