Added 'wpsc_product_min_cart_quantity' filter.

Allows you to set a minimum order quantity.

This implements #219

The way this currently works is if minimum order quantity is set to 10 and someone orders 6 of something, it will add 10 to the basket.

Is this logical or should it not add anything to the basket unless the user inputs a number >= the minimum value, rather then adding a rounded up number of items?

Currently not user feedback is given.