Added extra order detail parameters

Based on documentation:
https://developer.paypal.com/docs/classic/express-checkout/integration-guide/ECCustomizing/#id086NA300I5Z__id086NAB0M0UI

Extra order details can be passed to show more bits on the left.

This complements the items addition that was made recently in pull request https://github.com/omnipay/paypal/pull/7
(I was adding items and receiving a response about items not adding up, I needed a way to add the shipping rate, hence this PR).

Some basic testing and unit tests have been done / added. Please let me know if you find any issues.