RFC: Taxation without adjustments

[mamhoff]

Currently, we use the Spree::Adjustment model to save taxation data on line items, shipments and orders.

Adjustments have issues:

• They store labels in the database, making them hard to use across different locales
• They need a source and are recalculated every time the order changes. For taxation, that's not so desirable as we are moving towards doing taxation for the order all in one go.
• They are polymorphic

Here's a proposal: Add a new thing called Spree::Tax that works just like an adjustment, except that it is somehow translatable through I18n, cannot be used for promotions, and is tied to a Spree::Shipment exclusively.

If line items need taxation, they can have a line item tax estimate, too - as a separate model.