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
Checkout: pass through currency type

Add passed currency type before falling back to default.

Closes #1451.
make token exchange method public, use MultiResponse in transactional methods
Adds standard error codes on purchase and authorize of bogus gateway
Fixes #1450
[USAePay] added remote tests for track data and remove address/customer info in request
Fixes #1430
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
Payment Token metadata hash should be with indifferent access
Merge pull request #1455 from Shopify/token_hash_with_indifferent_access

[Bugfix] Payment Token metadata hash should be with indifferent access
Include PartnerID param in eway_rapid requests if set
added standard error codes to usa_epay
Merge pull request #1456 from Shopify/usa_epay_standard_errors

Usa epay standard errors
added standard errror codes to mercury
Merge pull request #1457 from Shopify/mercury_standard_error_codes

added standard errror codes to mercury