Adds preliminary support for Klarna

@bslobodin @louiskearns Here’s my first draft for Klarna support.

Plan:
- [x] Klarna ActionView helper with knowns and existing Shopify API
- [ ] Add support in Shopify for passing in cart items with the data that Klarna needs*
- [ ] Get confirmation on what testing and production service_url is
- [ ] Confirm that merchant_terms_uri, merchant_checkout_uri, merchant_base_uri, merchant_confirmation_uri can all be the same URL to the cancel page
- [ ] Confirm that it’s cool to do locale-guessing from just the shop country

*Cart item data that Klarna needs: 'type', 'reference', 'quantity', 'unit_price', 'tax_rate'