WooCommerce Xero Integration 1.9.1

* Fix - Fixed fatal error is thrown when marking an order paid via COD as complete.
* Update - WC tested up to version.

* Fix - Compatbility with Subscriptions where invoice not sending on renewal orders.
* Fix - Send invoice on order payment completion not working.
* Add - Filter to allow manipulation of the tax label name creation. "woocommerce_xero_create_unique_tax_label"

* Fix - Additional updates for WooCommerce 3.0 compatibility