8.0: add module sale_tax_included_excluded

This PR is the remplacement of Sebastian BEAU's PR https://github.com/OCA/sale-workflow/pull/46 on the same topic.

This module aims at solving the following dilemna: in OpenERP by default, you can't properly manage both prices with tax included in price and prices with tax excluded in price. It is an important need if your company does both B2B (tax excluded from price) and B2C (tax included in price, at least in Europe). You also have this need if you do only B2C with tax included in price and you have some foreign B2C customers that buy without VAT and you want to sell them with a lower price without VAT.

This module is the best option we found (after evaluating many different options) in order to handle both prices with tax included in price and with tax excluded in price. One of the goals of this module is minimize the impact on the datamodel of OpenERP and keep it small and simple. As we tried to keep the code as small as possible, we faced some issues with the function product_id_change of the official addons. So, in the short term, we supply a patch to update the code of the sale module and the product_visible_discount module. But, if this PR on odoo/odoo is merged https://github.com/odoo/odoo/pull/3717, I will be able to update the code of this module and remove the need to patch the official addons.