Postgres DB Support

[lfkeitel]

Implement support for the PostgresSQL database. Most of the structure is already in place. The following would be needed:

• Add option to <select> box on install page
• Add driver statement in the DB class
• Evaluate current SQL statements to see if they can be compatible with all three (MySQL, SQLite, Postgres) or if additional SQL statements will be required

[lfkeitel]

The new Repository system will make this much easier. All SQL is in separate repos per database. I just need to get a DB setup for development and edit the SQL to fit Postgres' flavour.