Shopping cart - 2nd submission.

I kept `still` here to emphasize the fact that the cart did not change due to the operation in `When` condition. True I can reuse the same step definition if I remove the world `still`. But IMO using the word `still` convey the message clearly.

Also we can write step definition like this, so that we can re-use the same step definition for multiple cases

`Then /I ?:((still|only)\s+)?see (\d+) units of (.+) in the cart/ do |num, item|`