Kraken authenticated

Added a few Kraken DTOs for public data. 

We couldn't figure out how to get authenticated. See https://github.com/omnibrain/XChange/blob/08d4e296c80213a9964d5cdf467d646e04709370/xchange-kraken/src/main/java/org/xchange/kraken/service/KrakenDigest.java . Any help is appreciated.