Handling a customer purchase

At first glance, adding a purchase seems like a trivial process. Looking deeper, however, it becomes clear that we need to somehow bring together information about the customer making the purchase, as well as the products to be purchased. 

Altogether, the process of adding a new purchase entails the following:

1. Authenticate the customer.
2. Select products and quantities to be purchased.
3. Add the purchase to the database.

Before we can accomplish all of this, however, we need to introduce three new classes that provide web support: web.session.Session, web.auth.Authenticate, and web.responder.Html.