Support XML-RPC services for account creation, validation

It would be very useful if Logintoboggan supported exposed an RPC service using the Services Module to register users and validate their accounts.

Here is a first draft implementation. It still needs a little work, but it would be great to get something like this incorporated into a future version of Logintoboggan.

The patch splits apart the UI and backend part of user registration and validation, and the .tgz file provides a Service implementation that uses the backend to create and validate accounts

In this implementation, all of the services take a version as the first parameter, which is currently ignored. Using the version field allows changes to the RPC interface to be versioned, so for example if we need to change the arguments or return values later on, we can do that while providing backwards compatibility by providing different behavior depending on the version parameter.

Feedback is welcome!