Improved layout of PHP module deps

- reordered modules alphabetically for better readability (for example when comparing with a repo package list), addresses my concern from #228 
- moved DB modules into own section

Also:
- added minimum postgreSQL version of 9, as it seems that the new syntax we use isn't supported in version 8 (see https://github.com/owncloud/core/issues/6294#issuecomment-32041954)

Please review @bantu @DeepDiver1975 @karlitschek