Harry Potter - testing no discount in single books

A simple test to ensure that a discount is not applied to single books.
Add test for harry potter 2-book discount

Add a test for discounting two books.
Add test for harrp potter 3-book discount

Adda test to ensure a 10% discount is applied to sales of three books
Add test for harry potter 4-book discount

Add a test for calculating the discount of four harry potter books
Add test for harry potter 5-book discount
Move methods into Checkout class
Add contexts to tests

Place each 'it' block inside a context block so that let blocks can be
used to set variables. This will allow the number of books to be updated
and passed into the subject.
Replace case statement with a hash
Split discounting into two methods