Fix registering a user

An attempt to address https://github.com/twisted/vertex/issues/14.

Fixes old AMP usage and adds a sorta test to make sure that `q2qclient.UserAdder` can successfully `callRemote` to `q2qstandalone.IdentityAdmin`.

Tried to use `loopbackAsync`, but `IdentityAdmin` depends on having a transport that has a method `getQ2QHost`, and `loopbackAsync` uses its own transport, which does not have such a method, and the transport doesn't get exposed for me to patch it.  So using `twisted.test.iosim` instead.

Also included a dependency on https://github.com/alex/pretend for testing.