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.
I reckon this value would make more sense as an attribute of `Spree::ShippingMethod`, since we might want different rates on different methods. It's also a good general principle to avoid global state unless the value definitely belongs there.