uSync.Umbraco.Commerce tax class fix + 17.1.0 release

Repo: Jumoo/uSync.Umbraco.Commerce

Fixed country/region tax rates not being restored on import — the serializer wrote them under a <TaxClasses> element but read them back looking for <TaxRates>, so import always saw an empty list and wiped existing overrides instead of restoring them (#10)
Fixed the tax rate value itself round-tripping as 0% — it was serialized via TaxRate.ToString() ("20.00%") instead of the underlying decimal, which failed to parse back on import
Fixed a related bug in ShippingMethodSerializer where a collection-diff compared the live country/region list against itself, so stale allowances never got removed on import
Released 17.1.0 to NuGet — first non-rc release of the 17.x line — with a matching GitHub release
Added umbraco-marketplace.json for the Umbraco Marketplace listing