- reduce poll iteration count
- change request URL for testing
- fix typo

- remove duplicate compile sources
- sync dynamic and static framework umbrella headers
- rename BUYClient+Checkout helpers category to avoid duplicate name

- add initializer to payment provider to inject fake safari controller

Also update for changes to how models are created, various other changes.

- add two classes conforming to BUYPaymentSessionProvider for internal use
- modify the API to only return payment tokens wrapped by protocol
- merge two `completePayment` methods into a single method