2 2 upgrade

hook onto order change events to trigger tax calculations.

we tried coercing avatax's order-level system into fitting spree's line_item-level system but it was getting ugly.

instead, provide a no-op calculator to make spree happy and create an order-level tax computer that hooks into order changes.

clarke took a look at this and said the approach seems as sane as it can be at the moment.  he agreed that long-term we should try to work with spree to make the tax system much easier to plug an external tool into.