Adding account vat on payment 7

Moved from https://code.launchpad.net/~domsense/account-payment/adding_account_vat_on_payment_7/+merge/196914
Absolutely, otherwise all your manipulations from then on will lose i18n.

Put this at the top of you onchange:

```python
    if context is None:
        context = self.pool['res.users'].context_get(cr, uid)
```