support multiple tax rates per invoice

[SubandiK]

fix #6

Adds support for multiple tax rates per invoice.
No changes required in the API, but Invoice.tax_rate is now obsolete. I left it in because you asked for backwards compatibility, so we don't throw an error if trying to set it. Now, numerical tax information is taken from the line_items, Invoice.tax_amount is still used for validation and for TaxTotalAmount

tax_category and tax_reason are still taken from the invoice:

• German 7% and 19% are both the standard category and I don't currently see a case where one (but not another in the same invoice) line item would have a different category
• tax_reason is simply not used by us. It could also go into the line_item object, but then we'd have to send it for each line_item - meh

Also, feel free to suggest better syntax, I'm much more of a Rails than pure Ruby person so if you don't like something by all means let's change it :)

[SubandiK]

I'm not sure how this works, or if I should have put my name in here? I personally don't care, but neither "signing in your name" nor "putting in my name here while I don't care about copyright at all" felt truly right.

[halfbyte]

I'll come up with something along the lines of "Copyright Secretariat Team and Contributors, see CONTRIBUTORS.md" or something.

[SubandiK]

actually don't review yet please, I have found a potential issue that needs fixing; will update

[SubandiK]

@halfbyte it now works as expected even if there are line_items with quantity > 1, adjusted spec too.