Locally maintained tax rates (OCC-36)

[sarahelsaig]

Part of #33

This is the part when you need international shipping, but don't want to use some cloud service

Site setting

Need a place to store the tax rates. We need the ability to match against multiple criteria like each part of the destination address (especially country and postal codes), or the TaxPart.ProductTaxCode. This could be implemented many ways. For example as an editable table containing the rules using DataTables (see Lombiq.DataTables), or implement a custom DSL using a parser framework like Irony and create rules in a multi-line text field.

Models and Services

Add an ITaxProvider implementation that uses the above setting to generate the tax amounts and gross prices.