Zone review

Here is my proposal of how the tax definitions should be structured.
1. Reverse naming i.e. vat_ch rather than ch_vat
2. TaxType to define a common tax type e.g. EU VAT or US Sales Tax. This is needed for things like Intra-community reverse charge and the new electronic services regulations.
3. Tax to define a rate e.g. an EU Country Rate or a US State rate.
4. Addition of a "code" this is a short code version of the tax name that can be used instead of the % value and is also used for the id rather than the full name.

If this is agreed we can set up all the other definitions