Updated Google Checkout and Amazon FPS. Added missing features, and fixes

GoogleCheckout now has Shipping and Tax support, it was missing before. I added some but not all of those features, I didn't have time to add everything, but hopefully the next person can see what I did, and finish the rest. 

I also updated the documentation and wrote some tests.

Amazon FPS was broken on the latest version of Boto, so I updated the code to work with it.

Let me know if you have any questions or issues.