pass through currency to checkout button

Currently the pay button always shows USD, even if the sellable changes it to e.g. GBP

I've changed it so the currency is passed through to handler.open, which will update the {{amount}} text in the pay button

see https://stripe.com/docs/checkout#integration-options-currency

![screen shot 2014-10-28 at 09 15 05](https://cloud.githubusercontent.com/assets/2088870/4805914/116b4990-5e83-11e4-987b-4ca9be6749ea.png)