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 thought about that, but would there be a case where the tax rate would be different on different shipping methods? In the UK you pay 20% tax on all delivery costs, and the same rate is applied to all deliveries. Seemed like setting the tax rate once and referring to the value when needed was better than setting the tax rate every time a shipping method was created. In the latter case, I think we'd want it to default to setting "20%" for the UK every time a user set up a shipping method... but then we'd have to store that default value somewhere...