WooCommerce Xero Integration 1.9.5

* Add - Apply the VAT/Tax Number from order to the linked Xero contact.
* Add - Filter woocommerce_xero_contact_tax_number for VAT/Tax Number assigned to contact.
* Add - Filter woocommerce_xero_vat_number_meta_key for VAT/Tax Number meta lookup.
* Fix - Error sending payment when order contained discounts due to a rounding error.
* Fix - Use billing address to calculate tax for orders with virtual items.
* Tweak - Send 'NONE' as Tax Type if WooCommerce taxes are disabled.

* Tweak - Remove legacy code.

* Add - Filter woocommerce_xero_disable_auto_void_invoices to disable voiding of invoices.
* Tweak - WC 4.0 compatibility.