Add Coinbase integration

Coinbase (https://coinbase.com/) merchant tool integration.

See previous discussion: https://github.com/Shopify/offsite_payments/issues/15
If we receive a request with a smaller nonce, the request is rejected. I agree there is a possibility for a race condition here, but it would only surface when there are 1000s of connections / second and it would not be fatal. The callback is retried automatically, so it would be no problem there, and the user would just have to refresh the payment page if it happened on the user facing part of the site. The time is of such high resolution that I think it is very unlikely this will ever happen, however, unless multiple servers are very far apart from each other in terms of time.