Take tax_category from Line Items and Fix validation: rounding for subcent amounts

[SubandiK]

Now taking the tax_category, which is set per tax rate, from the line_items.
This is needed because different taxes can be on one invoice, and they aren't all guaranteed to be of the same category
(see #16)

See also my comment from there:

This would also render Invoice.tax_category nearly obsolete, which is then only really used in tax_reason_text, which is only used at the per-tax level, so that also could be changed to not use invoice.tax_category

I kept that part in place because I wanted to change as little as possible, but in theory I could also do that.

Also changing the rounding in the validation in two places:

• on the invoice, use .round(2) instead of .round(2, :down). As far as I understand, in Germany there is no forced rounding down
• on the invoice item, introduce .round(2) as well

the resulting invoice passes schematron and various online zugferd checkers.

fix #13
fix #16

[SubandiK]

I changed the quantity here, because I wanted a quantity of 1 for the 7% item, to show the rounding problem, but still keep a test with a quantity of > 1 around.