Add unit withoutax
When taxes are configured like include in price, by default OpenERP on the price_unit field for account.invoice.line object display the price with tax included, but that is not legal on Mexico and don't know if there is an advantage somewhere.
This commit introduce a new calculated field for store the price without tax that is the one we needed on the PDF file. Later we are going to need to evaluate the convenience of include the same field on the account module.