extensions: add register-and-verify extension

The `REGISTER` and `VERIFY` commands provide a unified, standardized interface for account creation and validation to the network's authentication layer.  This allows for a network to provide a common interface regardless of what authentication layer they have chosen for their network to operate, i.e. traditional services, a different central authority, or a decentralized model similar to [IRCX][ircx].

   [ircx]: http://en.wikipedia.org/wiki/IRCX

We believe that the benefit of adding a common interface for account creation and validation, is helpful to the end user in discovering the capabilities offered by the authentication and authority layer of the network. Further, we believe it is helpful to client authors, so that they may provide guided account creation and validation for their users, regardless of network configuration.
Replace <email> with <callback>?