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Updated the AddToCart stored procedure. It will now check if the item to be added already exists in the shopping cart, and then increment the quantity by the amount if so. The stored procedure can also take negative values; if the product doesn't exist in the cart and the quantity is negative, no change will occur. If the product does exist and the new quantity after incrementing/decrementing the current quantity is less than zero, the product will be removed from the cart. (refs #92)