Patch to pdo

i changed pdo class. PDO class handle all older functionality of mysql class. 

On the other hand, I create a class and its constructor create an PDO object. we can extend from PDO class. Mine is more easy for me. 

To use this class as default database driver, you can change your DB_DRIVER like this: 

      define('DB_DRIVER', 'mpdo');

I added extra methods, `prepare`, `bindParam` and `execute`. These methods can be preparation for binding parameter from models. Maybe this can be a starting. Maybe opencart change their opinion about binding parameter to sql. 


I will try to write for mysqli...