Add BitGo wallet

This change adds bitgo to the wallet list.

BitGo has been in operation for 6 months now without incident.  BitGo operates similarly to blockchain.info in that the user holds the keys (e.g. bitgo is not a 'web wallet'), but Bitgo is different from blockchain.info in that wallets are P2SH/2-of-3 multisig.  You can read about how it works in our whitepaper at https://bitgo.com/p2sh_safe_address).  We've been helping the bitcoin community with open source patches for multi-sig work which have been accepted in bitcoinj (multibit and the android wallet), blockchain.info, and others.  Our core bitcoin multisig work is all open source and has been used by several of the follow-on multi-sig web wallets that have released this year (bitcoinjs-lib).  We have a full time staff supporting the wallet, and I believe is a good candidate for this list.  Please let me know if you have any questions.