Hitbtc: change sorting order for public trades 

This exchange requires you to specify varargs (unlike most/all others) to get any trades. Also, if we do not sort descending then you must first figure out a timestamp or an order id for which you want to filter from to get current trades (since otherwise it starts returning trades from December). With this change you can call:
```
marketDataService.getTrades(CurrencyPair.BTC_USD, 0L, HitbtcTradesSortOrder.SORT_BY_TRADE_ID, 0L, 1000L);
```
to get the latest 1000 trades. Also I don't think `start_index` is ever anything else but 0.