Class Cart: Manage the items of a shopping cart

This class can manage the items store in a shopping cart.

It can add items to a cart given the item names, quantity and price.

It can also update a given item by specifying its name. If the updated item quantity is zero, the item is automatically removed from the cart.

The class can also generate an HTML listing of items of all items in the cart and automatically compute the total price of all items.