Cart: Utility library for handling items added to a cart

Facilitating vat and discount calculations
Cart: Refined implementation for handling disounts properly

Distributing remainder or excess discount to minimize total rounding
error and effectively gurantee that exactly the total discount is
distributed over all line items.
Cart: Making proper assertions in 2 tests

Instead of a couple of System.out that where development remnants
Cart: Adding inverse method

Returning a new Cart with all quantities negated
Also some general formatting improvements
Cart: Removes redundant quantity on discountLine

As the quantity now is transported directly via the Discount interface

Also removed public modifier from constructor as this object should
never be instantiated by outside classes
Merge pull request #63 from iZettle/cart

Cart