Cart: group more values per vat group.

* changed the name for method groupValuesPerVatGroup -> groupValuesPerVatGroup
* groupValuesPerVatGroup will now return a object that contains actualVatValue and actualValue
for that vat group.
Cart: Method rename and formatting
Merge pull request #76 from iZettle/group-values-by-vat

Cart: group more values per vat group.