update regexp for parsing out valid bitcoin address

Bitcoin addresses are 27 to 34 characters long, can start with 1 or 3 (multisignature transaction). It also follows base58:

https://github.com/defunctzombie/bitcoin-address/blob/master/base58.js#L6

Would be nice if @dominictarr or @joates could test this on real iteration data. I've only tested the address matching part.