Reorganize private methods; add comments; consistent naming.

Simplify object -> JSON serialization.

Also fix a bug in `-buy_JSONForObject:` at the same time. Was incorrectly sending `-valueForKey:` to the wrong object (we already had the object who owns the identifier).

Add class declaration.

Move models to new folders

Update extensions to Cocoa Foundation classes.

Standardize category names and unify to have one category per class. Move files to "Additions" directory.

Migrate to App Extensions API.

- switch from channelID to appID
- adjust API endpoints for products and collections
- adjust mocked responses

Pagination Support for Collections

BUYLineItem didn't have a productId when initialized with BUYVariant

Update develop from master

Public/apple pay shop fix

Correct the spelling of CocoaPods in README

Also improve error description.

Adds bundle identifier to user_agent