Offline multi-signature HD wallets

This is a redo of pull request #266 for the 1.9 branch.

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 name _oms_)
*    multisig addresses are in sync between the three online and the three offline wallets (account _m/0'/0'_)
*    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

This is a somewhat different approach than the beginning of the p2sh effort in the current 1.9 branch.  The main difference is that three independent wallets are used, rather than using three branches from the same root.  The advantage is that the signing wallets are completely independent and there is no single secret that can compromise the multisig coins.