Simple Static Basket: Manage shopping baskets stored in sessions

This class can manage shopping baskets stored in sessions.

It can create and manage the items in a shopping basket using session variables to store the basket data. Currently it can:

- Add a list of items to the basket specifying the items' id, price, name and quantity
- Change items already in the basket
- Get the list of items in the basket
- Get the quantity of items in the basket
- Get the quantity of item types in the basket
- Get the total cost of the items in the basket
- Get the last modification date of the basket
- Remove items from the basket