Create new ExchangeRateProvider implementations for Bitcoin

[keilw]

After the example provider for MtGox has become irrelevant due to the exchange being out of business, it would be great to add some providers for any of these (still active) exchanges: https://en.bitcoin.it/wiki/Category:Exchanges

I use Bitcoin.de myself and it is noteworthy, because it is backed by Fidor Bank in Germany which adds a certain level of trust and stability compared to others. Happy to get the ball rolling, but contribution is greatly appreciated.