add ApplePay <-> Stripe Token exchange, change transaction method arguments 'creditcard' -> 'payment'
raise a ApplePayTokenExchangeError if token retrieval fails, adjust a few more method signatures to use payments vs creditcards
add remote tests for transactions using Apple Pay Payment Tokens
add unit tests and valid apple pay token exchange response
extract Stripe API requesting block from #commit into #api_request method
make token exchange method public, use MultiResponse in transactional methods
purchase and authorize methods should return a Response to maintain existing interface
add expects on apple pay tokenization methods in tests, move apple pay token fixture helper into test helper
#store should be able to process Apple Pay Payment Tokens
Merge pull request #1449 from Shopify/stripe_apple_pay

[Stripe] Add Apple Pay Payment Token exchanging