enh - added bip32 seed parsing

This patch allows the creation of bip38 keys for addresses created by
Richard Kiss' BIP32 pycoin (genwallet) utility.

    test vector outputs:

        $ python -c 'import binascii; open("master-private-key-entropy", "w+b").write(binascii.unhexlify("000102030405060708090a0b0c0d0e0f"))' > test.bip3

        $ genwallet -f test.bip32.seed -i
        xprv9s21ZrQH143K3LhEXZMXowo9hkNiaiDVhfvnGpXCimZtiRc5gJmq1YLKjLeg7iR2xDU8gks56oB1JXxwBzNkX5GHQtnjActTpWudHkpbgNu
        main network
        private key
        secret exponent: 64984153768961157829826368741579665624823626924420053033036842588453764406980
        public pair x:   68560981314052299508403304659895617070088830480209293291335193415198847031373
        public pair y:   90409367661552839451235119846362341380971283312313541569547197748756276715666
        tree depth:      0
        fingerprint:     a4ea9e94
        parent f'print:  00000000
        child index:     0
        chain code:      80bbda1ee5fdc3d94a217325d2fd7290cbd69cb91fc69dc8d21e84c9a7fe870d
        WIF:             L22zGSFr75Cix3h8BUKxfyRUTpxEnNC442pvjnwP2F2GNEppUKuc
          uncompressed:  5JuZTcBAK9CigCwQNrh6N4io6dEshdyRi1sHFgQecdFnd7eMcgL
        Bitcoin address: 1G2zt786BsvKJXAvTamW3Bds64e6pcQBMx
          uncompressed:  1PqxhQ4mJNUVUPoWGznMhqf33HGcfaonaH

    tested with above values:

        copy secret exponent, paste seed into PyBrainwallet
                64984153768961157829826368741579665624823626924420053033036842588453764406980

        check bip32 checkbox, uncompressed key and address will appear
                5JuZTcBAK9CigCwQNrh6N4io6dEshdyRi1sHFgQecdFnd7eMcgL
                1PqxhQ4mJNUVUPoWGznMhqf33HGcfaonaH

        check bip38 checkbox, encrypt with string 'secret', get bip38 key
                6PRSKsniRtueuhY2LM47EvXtfzKkAbrtsDasXhUpZgNBRqBdy56cHVLCMj

        import key using mycelium wallet, enter password 'secret'

        key successfully imported