This follows the coding style guideline that the file that contains a
class should follow the exact name of this (main) class.
pubkey.{h|cpp} -> PublicKey.{h|cpp}
pubkey_utils.{h|cpp} -> PublicKeyUtils.{h|cpp}
This introduces a Mnemonic class for BIP39 (seed words)
validation as well as a HDMasterKey class with provides the
BIP32 & BIP43 support.
All tests went into the apputils unit test.