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.
@rohanM I think in our case the default tax rate would be set to 0% and the 20% tax rate would be applied to some, but not all products... Also there's no indication (from a user's perspective) that it grabs the default rate and applies it to shipping. It seemed like being able to set them separately was a good idea.