Added DigestUtils with static method bytesToHex
[Vircurex] Removed DatatypeConverter

Class is not included in Android, using new method from DigestUtils.
[HitBTC] Moved bytesToHex to utils
Changed default trade id to null

HitBTC clientOrderID is generated depending if the trade id is null, so
clientOrderID was being sent as "".
[HitBTC] Improved error handling

Added to placeLimitOrderRaw