Use bip32 for key derivation

[fjarri]

Instead of a homebrew derivation, use BIP-32 standard.

There are some future concerns:

• Generalizing this to any curve as a part of Generalize the library to use any curve #27. bip32 will have to be generalized for that as well, since as of 0.5 only supports secp256k1.
• Initial chain code derivation. See What is the best way to derive the initial chain code for BIP-32? #134.
• Only non-hardened derivation paths are supported, and there is no forward secrecy. See Hardened derivation and forward secrecy #135.