Fix for #20: Add missing Postgres-PDO

Hi,

this PR contains the missing php-package for postgres-support as well as an additional normal (non-root) user for postgres.

As suggested by @josegonzalez  this users name is `username`.

I'd like to mention that, for (mysql-)harmonized credentials, the username could have been `user`, but as this is a reserved keyword in postgres, I went with `username`.