Making CART_MODEL_DB variable optional in settings

Not having to declare _CART_MODEL_DB_ in settings seems more practical. It also adds backward compatibility with older version of session_cart.
I was using _Curtis Maloney_ version before using your's, and an exception was raised on upgrade.

This is not an issue, just a different design point of view.

Also, I'm making a pull request because I couldn't find an easy way to simply tell you, inside github.

Have a good day!