Add HDPrivateKey class
Add tests
Fixes interfaces
Add public key
100% test coverage for HDPrivateKey
100% test coverage on hdpublickey
Add cache to derivation
Fix style: move each static function to an appropiate namespace
Add missing javadocs
Change errors
Add autogenerated errors
add testing for key cache
Merge pull request #577 from eordano/feature/hdprivate

Refactor BIP32 -> HDPrivateKey and HDPublicKey