Add tax categories to shipments so that they are taxed correctly
Move Spree::Zone class methods to the top of the class

This is where I expect them to be
Show tax amounts within shipping rates
Add a test for tax rates adjusting twice in state_machine_spec
Remove double looping over tax adjustments in checkout/summary
Correct reference to tax category for Shipment
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