Cart should not allow negative quantities

[EvanDotPro]

Discussed w/ team, and @bjyoungblood maintains that the cart should actually allow negative quantities and prices, as it can be useful for keeping track of RMAs and stuff like that, and that it should be up to the application using the cart to decide when to restrict that. In this case, for the time being at least, we should restrict to positive quantities only in the catalog's usage of the cart.