Also improve error description.

This adds a attributes to checkout. These objects are returned as an array of dictionaries, but expected as dictionaries when posting the checkout, so we're required to serialize the objects correctly. API now also returns the `note` on a checkout, so I have added a test for that.

cleanup error handling for Apple Pay
Use dispatch_groups to ensure the shop and checkout are completed before continuing with an Apple Pay checkout

Move logic for displaying the Apple Pay setup button to the BUYViewController, and fix logic for whether to display Setup or Buy buttons.

Use pathWithComponents: and stringByAppendingPathExtension: for better consistency with URL creation (+4 squashed commits)
Squashed commits:
[90e8970] Also use pathWithComponents: for "checkouts" with token and fixing tests by appending a slash to "api" and "meta"
[4eb36a4] No longer appending = for purposely blank values
[4c0f4e2] Improve method names, use of constants and appending .json when assembling the NSURLCompenents path at the end
[935cbb2] forgot to stage /meta

Improve method names, use of constants and appending .json when assembling the NSURLCompenents path at the end

No longer appending = for purposely blank values

Also use pathWithComponents: for "checkouts" with token and fixing tests by appending a slash to "api" and "meta"

Use pathWithComponents: and stringByAppendingPathExtension: for better consistency with URL creation

"/anywhere/checkouts" is now "/api/checkouts". This transition has been made using NSURLComponents. Tests have been updated to better match testing of the scheme, host, path and query items from which a NSURLComponents URL is created.

Fix an issue where partialAddresses flag was set to false, which would invalidat the already set (through a updateCheckout: call) shippingRate on a checkout during completeCheckout:

Refactor partial address handling to prevent a partial address to be applied on a completed checkout

fixes for integration tests

fix partialAddress unit test

Allow for partial address flag to be set to false on update

Prevent partial address from being sent post Apple Pay authentication