Added persistence for Users

Implemented some basic persistence (User details including username;
password; forename; surname etc.) in the form of an XML file.
Added code to the UserRegistry to instantiate these persisted Users.
Obviously, this is not a fit-for-purpose storage solution but is more
than adequate for development and testing. The likely scenario will be
to eventually replace the XML with a database (or simply introduce some
encryption to the XML file?).
Also - some further development to classes in the Users package.