Add Coinbase integration

Coinbase (https://coinbase.com/) merchant tool integration.

See previous discussion: https://github.com/Shopify/offsite_payments/issues/15
The information in the URL after Coinbase redirects you cannot really be verified until you get the callback, so I think it not a good idea to include it here as it would encourage bad practices. The payment is not necessarily complete when the user is redirected back to your success page.