Fiat, Monetary and ExchangeRate

Introduce Fiat as a holder for fiat values. Coin and Fiat share an interface Monetary so that monetary infrastructure can be re-used.

Adapt CoinFormat to be used with Monetary. Add an ExchangeRate value class that can convert from Coin to Fiat and back.
Java already has a class for this:

http://docs.oracle.com/javase/7/docs/api/java/util/Currency.html
http://developer.android.com/reference/java/util/Currency.html