* Fixed WSOD due to 'Call to a member function get_id() on boolean ...' when no subscription parent exists.
* Fixed possible exception on invalid order.
* Improved the subscriptions display so info is displayed also for subscriptions without an order.
* Updated usage of some methods and properties to avoid notices.
* Updated the plugin for compatibility with WooCommerce 3.0.0.
* Moved plugin boot process hooked on plugins_loaded to simplify version dependencies.
* Replaced use of properties by new WC 3.x getters/setters.
* Maintained backwards-compatibility within own library loaded version-specifically.
* Updated the product panel hook to use new action with backwards-compatibility.
