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.
Sounding good to me so far. @kirstenalarsen, @oeoeaio , can you add any perspective on shipping tax we're likely to encounter (in other countries)? Would a flat percentage site-wide be sufficient, or do we need per-shipping-method tax rates?