Account for situations where more than one tax rate can be applied at once
Add pre-tax amount to line items and shipments
There's more 'included in price' taxes than VAT (i.e. Australia's GST)
Remove test for tax_category in default_tax_spec

The check for relevant tax categories is now done in the TaxRate model
Correct default tax promo total spec
Calculating shipment + line item tax in DefaultTax is now combined into 1 method

Rather than two methods which do the exact same thing, just with different object names
Use tax rate's calculator to calculate shipping rate's tax