Added `group_pricing_policy` to `FixedRateTax`

Using a strategy derived from the `FixedRateTax` mixin breaks the default templates (which require a `group_pricing_policy` attribute).  I basically copied the one from `NoTax` and added the tax calculation from `FixedRateTax.pricing_policy`.
Merge pull request #1324 from mexicat/patch-1

Added `group_pricing_policy` to `FixedRateTax`