Base58 using sjcl.bn

My attempt at #155

This is a "Bitcoin" style Base58 (https://en.bitcoin.it/wiki/Base58Check_encoding)
* Uses the characters `123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz`
* Leading zeros are encoded to 1's

It works, but performance might be an issue... I'm open to suggestions. I tried to stick with what was already available in sjcl.