Discount per item

Consider this as a suggestion for how we should handle item specific discounts. Appreciate your thoughts @mikaellothman and @softarn !


I'll try to explain what I mean:

Imagine that we have an item that costs 10,00 kr and a discount of 1,335 kr. If we do the rounding as proposed in this PR, we would get 8,665 kr rounded to 8,67 kr. 

On the other hand, if we are about to print out the discount on its own line it would be rounded to 1,34 kr, right? In that case we would have 10,00 kr - 1,34 kr, which will be 8,66 kr.

I don't know what's right here, but it will look weird if we print receipts that doesn't sum up... 

And some additional confusion: Should the rounding be half-up-away from zero for discounts? :)