Add convenient method to add user to Realm

[olamy]

Implementation notes from @janbartel

• refactor the PropertyUserStore into UserStore base class that just stores knownUsers and has method to add a user like addUser(String name, Credential credential, String[] roles), and then PropertyUserStore would extend UserStore and feed it users from a property file. That way, either jetty or other projects could create eg an XMLUserStore to read the user info stored in xml files instead.
• refactor HashLoginService to have a setter to pass in the UserStore to use. For backward compatibilty we could keep the setConfig(String filename) method and autocreate a PropertyUserStore.

[olamy]

Cannot assign to myself but I will work on that :-)