Added support for ID Site Logout

I think we need to change the argument type of this to be a RegistrationResult (that would subclass the AccountResult class).  This is important to ensure that any registration-specific data that might be returned in the future can be represented without breaking method signatures.  i.e.

    onRegistered(RegistrationResult result);

Even if it is an empty subclass for now, that's ok (not ideal - empty subclasses are usually a yellow flag), but we can make this design concession now to ensure we're not put into an odd position in the future.  This has happened in the past - I want to ensure that we don't put ourselves in that position again.