Cartridge - Built in Taxation

The goal is to mimic shipping_type/shipping_total but have a tax_type/tax_total that provides the additional line item named, by default, "Tax."

"Tax" will show up in invoices, the Order admin, and on the order_totals widget.

This patch includes a migration script to add tax_type/tax_total to the Order model.

Like shipping, there is a tax_handler function defined by the setting SHOP_HANDLER_TAX which defaults to 'cartridge.shop.checkout.default_tax_handler.'
Also like shipping there is a new function 'cartridge.shop.utils.set_tax' that is used to set the tax amount in the session.

This patch has not been tested when used in conjunction with cartridge-tax.

This patch will play nice with google analytics tracking of transactions.