Adding account vat on payment 7

Moved from https://code.launchpad.net/~domsense/account-payment/adding_account_vat_on_payment_7/+merge/196914
Please split this script into one script per method/class:

* account_voucher.py
* account_invoice.py
* account_move_line.py
* account_account.py
* account_tax_code.py
* account_journal.py

This script is already ridiculously long, this is exactly why we should split by model.

I'll be able to do a better review after that.