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.
Ruby and Rails provide a number of methods for checking the presence or absence of a value. Here, `shipping_cost.nil?` would be more idiomatic than `== nil`. There are also `any?`, `present?`, `empty?`, `blank?`.