Support zero-cost items and item totals

Paypal Express does not support having zero cost items, or zero cost
item total. This is handled in two ways.

1. If there are some non-zero items, they are displayed through paypal
and the others are ignored.

2. If the items total is zero cost (not including shipping/taxes)
paypal is not given a detailed order breakdown, which results in it
simply displaying "Current order".

Fixes #10