Merge branch 'runmad.checkout-attributes' of github.com:Shopify/mobile-buy-sdk-private into runmad.checkout-attributes

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.

Prevent race condition during Apple Pay Checkout

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

Remove shippingRateId on BUYCheckout

Add help section + section links to readme

Remove province requirement from client-side address validation

Release 1.2.4

Remove unused orderId property

Fix Apple Pay "Setup" button logic

Sample app Safari view controller checkout logic improvements

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

Add Docset to repo

NSURLComponents improvements

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

Updated API endpoints

Minor improvements to the logic in the sample app for handling a SFSafariViewController checkout and returning from a successful checkout using the urlScheme button

"/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.

1.2.3 release version

Fix shipping rate invalidation bug using Apple Pay in 1.2.2

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: