Added support for Kraken.com trade and ledger data

[7cada]

Add Kraken support

[probstj]

The Kraken support added is fine, thank you very much!

But I don't understand how the Bittrex format can be so different than when added by anson-vandoren. Did they recently change the format? @anson-vandoren could you please also check the new format? We might need to add two different functions for adding Bittrex csv to not break the import of the old csv format.