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
While here, fix a bug found through testing. Register will now return
ErrDuplicateNet if the caller attempts to register any of the standard
network parameters provided by this package.
ok @davecgh
