Extend offers to work in both US and UK style tax regimes

Following an inspired suggestion by @izidormatusov, the price object has been extended to define an "effective price" which is the one used by offers.  This allows a single offer to work in both the UK and US.  Previously, we had a field that specified which type of price to use - this does away with that.

I've also corrected how offers apply to shipping discounts in regimes where tax isn't known.