Improve how discount amounts are distributed

Added property on DiscountLine: effectiveDiscountAmount

Total discount amount is now distributed over all discount lines in a
similar fashion as when done over items: to make sure that the full
amount is distributed while minimizing rounding error.

This property is now also used to calculate `effectivePercentage`

Added test to verify new behavior
Cart: Adds gross VAT amount on ItemLine and Cart

For easy retrieval when asking for the non-discounted VAT amounts

Also to be able to easily answer the total discounted VAT
Merge pull request #74 from iZettle/improve-discount-distribution

Discount distribution