Added support for inverse tickers.

Now the ECB integration derives the inverse direction of currency exchange.
So it's able to support also the exchange rate from other currencies
to EUR and not only from EUR to other currencies.
Merge branch 'master' of https://github.com/fairmat/MarketDataProviders

Conflicts:
	EuropeanCentralBankExchangeIntegration/EuropeanCentralBankIntegration.cs