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.
It's nicer (more efficient and also more idiomatic) to use symbols as keys for a hash in a situation like this. Like so:

```ruby
totals = {items: 0, items_total: 0.0, taxable_total: 0.0, sales_tax: 0.0}
```

Then below, accessing the hash will be like:

```ruby
totals[:items] += line_item.quantity
```