support segwit constants

[fanatid]

new structure versions (base58 prefixes) for bitcoin mainnet:

{
  address: {
    p2pkh: 0x00,
    p2sh: 0x05,
    p2wpkh: 0x06,
    p2wsh: 0x0a
  },
  privateKey: {
    compressed: 0x80,
    uncompressed: 0x80
  },
  bip32: {
    private: 0x0488ade4,
    public: 0x0488b21e
  },
  bip44: 0x00
}

[jprichardson]

I'm open to this, but I think it makes sense to get in segwit support first and then revisit structure.