keys: Expose HD tooling

[ebuchman]

We need to be able to recover private keys from the words written down in the fundraiser, which used HD (bip32). We already have the HD code under keys/hd but we don't expose it in any tooling.

We should have some way to specify an HD path when generating a secp256k1 key (note this only works for secp for now, not ed25519).

This is specifically to facilitate offline transactions, eg cosmos/cosmos-sdk#966

[liamsi]

possible via #118 (now living in cosmos-sdk).