Support P2SH and married wallets

[rmhrisk]

You can read about married wallets here - http://www.johba.de/married-wallets/

This will allow mobile wallets based on this library to be used in a way that they are resilient to phone loss. E.G. phone generates two keys, one to paper one for runtime and a third on a server. The server can then help the user recover and help them prevent their keys from being stolen in the case of a lost phone.