[FIX]chart of account wizard: show company id field in wizard view
[FIX]purchase: in a PO line, description of product should take into account product supplier name and code
[FIX] account_voucher: complete fixing of vouchers in multi-currency.

*uniformized the computation in multi-currency in order to always use res_currency.compute()
*make sure to pass the right conversion rate to _current_rate() function using the context if it is specified on the voucher and, in this function, used the info from the context instead of the computed one if relevant. (This involves an ugly overwriting of the 'rate' field that will disappear in trunk: to stay backward compatible and avoid any update of 'base' module it has to be done this way).
*replace the currency_id field on the voucher by a fields.function (was a fields.related) in order to always have a currency, even if it's a voucher in the company currency. (makes the treatment easier and more consistent. Less error prone)
*misc code reaftoring
[FIX] account_voucher: typo
[ ] empty commit to trigger a build in runbot
[ADD] account_voucher: new test for specific payment_rate specified on the voucher
[FIX] account_voucher: fixed new yaml test
[FIX] account_voucher: replaced xml ids in new yaml test to avoid confusion in yaml import that is breaking the test
[FIX] purchase: onchange of product now use information in the supplierinfo of the product for name and description if it exists
[FIX] account: mandatory company_id not present in the config wizard if already configured for this company, and thus it renders the view invisible as the company_id is empty and not visible
[FIX] account_voucher: using a new partner for the new yaml test to avoid interferences in the other yaml tests
[MERGE] account_voucher: fixes related to multi currency. All problems related to multi currency should now be completely solved :-)