Add special rounding for cash in stores (that don't use all the coins)

Some countries are allowed to not use all the available coins within a currency when dealing with cash. For example in some European countries, like the Netherlands.

See http://www.dnb.nl/en/payments/euro-banknotes-and-coins/rounding-to-multiples-of-5-cents/ and http://en.wikipedia.org/wiki/Swedish_rounding.
