Added possibility to specify superclass in dbus-proxy.el
* dbus-proxy.el (dbus-proxy-make-remote-proxy): added argument
  proxy-object-class; pass the class proxy-object-class to
  dbus-proxy-ensure-object-class
  (dbus-proxy-make-object-class): added argument proxy-object-class;
  when given, use the specified class instead of
  dbus-proxy-remote-object
  (dbus-proxy-ensure-object-class): added argument proxy-object-class;
  use as fallback