Add Base58 support

[craSH]

Base58 support (human readable encoding without the ambiguous characters 'O', '0', 'I', 'l') would be nice. It is used in bitcoin for public address hash encoding, and private key encoding.

Could use their MIT licensed code which is here: https://github.com/jgarzik/python-bitcoinlib/blob/master/bitcoin/base58.py

I may take this at a later point in time, but wanted to track it somewhere (you can assign it to me).

[jdukes]

Coding guide lines have changed, so if you make a change please make sure you add a hypothesis test for it.