changed how open orders at Kraken are fetched and parsed

before: code parsed a description string.
problem: special order types, such as "take profit limit" or "stop loss
  limit" caused NumberFormatExceptions because the format of the
  description string differed.
  
solution: Kraken API delivers all relevant information about an order
  nicely split up as a JSON object, in addition to the description
  string. So, just take the data from the JSON object.

not so nice yet: Special order types do not match to a LimitOrder, so
  we ignore them for now.
Merge pull request #282 from mhillenbrand/kraken_open_orders

changed how open orders at Kraken are fetched and parsed