Fix space before method declaration parentheses.
Define subtraction and division for Currency.
Add a Converter object, who holds exchange rates among Currencies.

It currently maintains a map of maps. The key of the outer map is the
starting currency, and the inner map is keyed by the ending currency.
The result of the double lookup is the rate of exchange between the
starting and ending currencies.
Define a method for converting from one Currency to another.
Add spacing between US CurrencyZone definitions.
Add CurrencyZone objects for Europe and Japan.