Options to add a secondary tax to invoices

Many countries, including Canada where I live, add two different taxes to invoices. First a federal tax and next a state tax. These two taxes need to be displayed on their own line. That's what this pull request is for. It's not very DRY, the best would have been to create a tax object and add the ability to stack many taxes on an invoice but we would have lost backward compatibility. Let me know if this can be merged.