Adding account vat on payment 7

Moved from https://code.launchpad.net/~domsense/account-payment/adding_account_vat_on_payment_7/+merge/196914
Sorry, cannot approve such a large method without test at the very least. If it's copied from some super method. Then I would like to see explicitly where it differs and suggest the original method is monkey patched (with new hooks made totally explicit) or patches in v8 so that it can be extended in a more modular way.