Sales tax report and config option

This is for issue #271 

-Added a new sales tax report to show tax due on orders in a given date range.
-Added a config option for adding/setting the tax rate on shipping.
Then again, I see that Spree has a global setting `shipment_inc_vat`. So in that sense, `shipping_tax_rate` belongs alongside it.

Looking at `app/models/spree/shipping_rate.rb`, I see that Spree expects that the shipping tax rate will be `Spree::TaxRate.default`. Do we need to introduce this variable because that's not always correct?