Adding account vat on payment 7

Moved from https://code.launchpad.net/~domsense/account-payment/adding_account_vat_on_payment_7/+merge/196914
As XML files can draw the odoo user interface, they should be treated as source code at the level of python files, I think.
I mean, if someone wanted to make the copyright notice explicit within XML files, I would not prevent it.