Base currency and counter currency in CurrencyPair named correctly.

Accroding to #34, I switched them names and added full code coverage on CurrencyPair.
There was a bug in CurrencyPair::getBaseCurrency() and CurrencyPair::getCounterCurrency() (they returned invalid currencies - getBaseCurrency() returned counterCurrency and getCounterCurrency() returned baseCurrency) which this PR fixes.