* Fix - Calculate last day of month correctly when sending invoice to Xero
* Fix - Ensure sending invoices on order creation works for order statuses besides processing
* Fix - Fix bug where same due date for new invoices was being reused for 1 month
* New - Cache invoice default payment terms for 1 hour
* New - Calculate due date for each invoice individually using cached payment terms
* Fix - PHP7.1 exception when sending invoices to Xero
* Fix - WooCommerce 2.7 compatibility
* Fix - Retrieve tax rates properly from Xero
* Fix - Instead of overwriting tax rates on Xero with the same name but different rate, create separate rates
* Fix - Use tax type EXEMPTEXPENSES when shipping is tax exempt for an Australian store
* Fix - Instead of generating a discount line item, apply discounts on the line items themselves to avoid tax adjustments
* Fix - Remove the discount account setting since we are applying discounts on the line items themselves
* New - Add payment method to invoice reference field
* Fix - Filter special characters out of description before sending invoice
* New - Add link from Xero invoice view back to order on merchant host
* Fix - Uniquely name new tax rates as rates change (Xero does not allow multiple active rates with the same name)
