Renamed CurrencyPairConfiguration to FXRateConverter.

Each FXRateConverter has the potential of being used by multiple currency
pairs.
Started implementing CurrencyPairConfiguration.

This really is to replace the Map<CurrencyPair, FXRateConverter>
InMemoryMoneyChanger needs for its constructor.
Implemented getFXRateConverter to return the given converter.

Instead of just returning the default one.