Improve HitBTC

Added HitbtcException to handle error message responses

Fixed trade adapter for cases where the trade id is null

Amended digestParams to remove need for DatatypeConverter
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
Format
Merge pull request #661 from zholmes1/develop

Improved HitBTC and Vircurex