Offline multi-signature HD wallets

This pull requests builds on pull request #212.

An offline multisignature workflow (OMS) is implemented.  This allows 2 out of 3 offline machines to sign a multisignature (p2sh) output.

Features:

* multisig addresses are generated deterministically in a separate sequence (account #1)
* multisig addresses are in sync between the three online and the three offline wallets
* The offline wallets auto-sync when transactions are signed, so that change can be identified
* The offline signing process displays the address and amount of the outgoing output for verification
* Examples scripts are provided for setting up the OMS accounts on each seeded and seedless wallet