Add Coinbase integration

Coinbase (https://coinbase.com/) merchant tool integration.

See previous discussion: https://github.com/Shopify/offsite_payments/issues/15
According to the [docs](https://coinbase.com/docs/api/authentication),

> The nonce is a positive integer number that must increase with every request you make.

What is the definition of "must", do you know? What happens if Coinbase receives a request with a nonce that's smaller than the previous request? I'm thinking not all servers will have an identical clock even with NTP and even on the same host there's no guarantee of FIFO from multiple application instances.