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.
You could use LongMath to do this:

http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/math/LongMath.html