Fixed installer bugs + Old MySQL code & PHP 5.3 compatibility

I also set mysqli as the default driver and updated the hard-coded code in the installer to use the mysqli.db.php class instead of the mysql class.

I also removed the mysql option as it is now deprecated. Also, I set the installer to check if the server uses PHP 5.3+ instead of 4.3. You can change that to 5.2 if you want, although it isn't recommended considering other forums use 5.3 as the minimum PHP version.