Move to use PDO for database -- Mysql sqlite pdo

As per mailing list discussion - this removes duplicate code in database_library.php and database_library_sqlite.php and uses PDO for the underlying database connection.

Updated : 

 * config.php - now checks to see if the database is configured/setup, and attempts to redirect to /setup (for mysql) or perform an sqlite auto-intall.
 * database_library.php - changed to use PDO. Added function database_is_setup().

Note - this has currently only been tested against SQLite.