Add Coinbase integration

Coinbase (https://coinbase.com/) merchant tool integration.

See previous discussion: https://github.com/Shopify/offsite_payments/issues/15
```ruby
(byebug) post
"{\"order\":{\"id\":\"5RSG9R0F\",\"created_at\":\"2014-06-27T09:23:24-07:00\",\"status\":\"completed\",\"event\":{\"type\":\"completed\"},\"total_btc\":{\"cents\":1549,\"currency_iso\":\"BTC\"},\"total_native\":{\"cents\":1,\"currency_iso\":\"CAD\"},\"total_payout\":{\"cents\":0,\"currency_iso\":\"USD\"},\"custom\":\"3\",\"receive_address\":\"1DWRUP93FpYK3VptAtsd7PyNHDM7MD5aam\",\"button\":{\"type\":\"buy_now\",\"name\":\"Shop One - #3\",\"description\":null,\"id\":\"634a801e9832408783f1937d6a3bc11c\"},\"refund_address\":\"1JhzJK1QxWHf9Pua87cAmm4abUZmCyAEk3\",\"transaction\":{\"id\":\"53ad9a83f12b3b67fc000003\",\"hash\":null,\"confirmations\":0}}}"
(byebug) parse(post)
"order"
```