Update Installer To Focus On Available Databases Not Drivers (Add Experimental Support for PostgreSQL)

Instead of asking the user which database driver they would like to use to connect to their database, ask them which database server they would like to store their database in (MySQL or PostgreSQL). The installer then selects the best driver based on what's available.

If the user chooses to use MySQL, the order of choice is:
* PDO (`mpdo.php`)
* mysqli
* mysql

If the user chooses to use PostgreSQL, the order of choice is:
* PDO
* pgsql (`postgre.php`)

I've added support for PostgreSQL and labeled it as **Experimental**. Unfortunately for opencart, the current db schema doesn't map very well into Postgres. It's something I'd like to fix (and have help fixing).

The installer completes successfully for both MySQL and PostgreSQL database backends. I only attempted the upgrade with MySQL and it also appeared to complete successfully. 

**Note:** The diff is huge because I've added a modified version of the `opencart.sql` dump file for postgres called `opencart-postgre.sql' and it's almost 8,000 lines long itself. The actual diff is quite manageable to read and understand.