Provide XML-RPC services for managing user relationships

Providing XML-RPC services via the Services Module would be very useful.

Attached is a first draft of an implementation. It still needs a little work, but it would be great to get this feature in a future release of User Relationships.

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!