* Tweak - Add a note when applying costs for previous orders is running in the background
* Tweak - Use WooCommerce 3.0+ methods to get total refund costs instead of a direct query
* Fix - Ensure costs are applied to subscriptions as well as orders when triggered manually
* Fix - Address a conflict with WooCommerce Multistore plugin
* Fix - Ensure orders placed count is 0 instead of 1, when no orders have been placed
* Misc - Add support for WooCommerce 3.6
* Misc - Require PHP 5.4+