Adding support for PUBKEY_ADDRESSes not 0.

[CoinWhisperer]

These changes allow encryption/decryption for any bitcoin-derived coin, using the vbyte arguments of bip38_encrypt() and bip38_decrypt(). The vbyte is usually found in base58.h as PUBKEY_ADDRESS. For example, the PUBKEY_ADDRESS for litecoin is 48. The code is fully backwards compatible because vbyte has been made an optional argument.

Example usage:

>>> vbyte = 48   # litecoin address
>>> encrypted = bip38_encrypt(wif_key, password, vbyte)
>>> decrypted == bip38_decrypt(encrypted, password, vbyte)
>>> assert decrypted == wif_key