Shopping cart - Iteration 2

In this second iteration, I made the changes suggested in the review for Iteration 1. The major changes are the following:

1. Asserting the total cost of the items in the shopping cart throughout my Features.
2. Using scenario outlines for `coupon.feature` and `shipping_estimate.feature`.
3. Testing edge cases throughout (ie, adding more than one coupon; adding items now out of stock, multiple products in cart, etc).

Thanks!
What would your step definition be like if you were checking if 3 hammers were in stock?

I would use integers (1) instead of "a" here.  It makes writing step definitions easier without becoming syntactically incorrect.  As-is, either you would have to account for "a" being used as a variable equivalent to 1, or you would have to write two nearly-identical step definitions for "a" and for integer values.

`And 1 hammer is in stock`