Cart: Formatting

Applying consistent 1-tab indentation rule for all types of indentation.
No logical changes.
Cart: Add toString method on Cart object
Cart: Change type of vatPercentage from Double to Float

Not even France will need a full Double for specifying VAT percentages
Cart: Fix discount bug

When only one or the other of amount or percent was specificed, a NPE
was thrown.

Also adds tests to verify the fix
Cart: Group effective VAT amounts

Adds utility method for producing a summary of the amounts per different
VAT rate within the cart.
Merge pull request #72 from iZettle/cart-various-fixes

Cart various fixes