Add priv and public key bytes for HD wallet keys.

This commit introduces an HDPrivateKeyID and HDPublicKeyID field to the
params struct which are used by hierarchical deterministic extended keys
as defined by BIP0032.

In addition, a new function named HDPrivateKeyToPublicKeyID has been added
to allow the caller to get the associated public key ID given a private
key ID.

The tests have also been updated to maintain 100% test coverage.

ok @jrick