Code review comment for lp:~icsergio/openobject-italia/61_imp_withholding_tax

Improvement1: this implementation uses tax object's and there's no neeed to type the amount of the tax, as it's calculated.
The payment term has to be set in the tax code, together with the flag on the boolean field "withholding tax".
Improvement2: moreover, the module sets the tax expiration, at the time of payment of the linked invoice, which is the relevant time for the law.