Adding PostgreSQL support

Here are my changes to add PostgreSQL support to Mura.

Tomorrow I will write a few notes for you to explain some of the things to bear in mind with PostgreSQL.

I am more than happy to help with any PostgreSQL compatibility moving forward...

For testing, if you are on Macs the easiest way is to download Postgres.app - just launch the app and you have a fully functional PostgreSQL server running.

-- Seb