Add WP JSON API Library
Fix strict standards warning
Modify WP_JSON_Server to allow authentication
Add required global functions for WP_JSON_Server class
Add REST API settings stub
Remove unused WP_JSON_Posts class
Add REST API endpoints/methods to WC API class
Add new /checkout/add-payment-method/ endpoint

Part of #3396
Add add_payment_method_action() handler

Part of #3396
Add form-add-payment-method.php
Fix up markup on form-add-payment-method.php

So that checkout.js correctly displays payment method's fields.
Part of #3396
Only display add_payment_method() when logged in

Part of #3396
Improve form-add-payment-method.php error & button

Part of #3396
Call add_payment_method_action()

Part of #3396
Redirect to My Account page after adding method

Part of #3396
Add a notice that payment method has been added

Part of #3396
Filter gateways for add_payment_method form

Part of #3396
Fix "non-static method called statically" notice
Add REST API authentication class
Add REST API base class