[IMP] website_sale: add vat field on checkout
[IMP] website_sale: if base_vat is installed, when the use try to validate checkout, the vat is checked (VAT Information Exchange System)
[IMP] website_sale: use company vat_check_vies field to select the method to check the vat id base_vat is installed
Merge pull request #226 from odoo-dev/master-website_sale-add-vat-chm

[IMP] website_sale: add vat field on checkout + validate it if base_vat module is installed