Implemented the behavior to use buyRate when amount is < 0.
Started implementing InMemoryMoneyChanger.
Implemented CurrencyPair.

Stopped implementing InMemoryMoneyChanger for the time being.
Made CurrencyPair.getInstance to memoize currency pairs created.
Refactored InMemoryMoneyChanger to internally use CurrencyPair.
Ignored tests for getting rates of inverse currency pair.

MoneyChanger needs to accept a configuration that specifies the
number of decimal points of the rates, especially inverse rates.