Added specs for products
Added product
Added checkout tests
Added Rakefile with test task

Just run `rake` or `rake test` to run the test suite
Checkout first approach without promotional rules
Added promotion tests and code
Added promotional discounts to checkout#total
Added 10% over $60 discount promotion to tests
Added bulk price per item promotion rule to checkout tests

* Promotions more flexible with variable arguments
* Added checkout#item_amount tests and method