Made emission of logging messages configurable in dbus-proxy.el
* dbus-proxy.el (dbus-proxy-enable-logging): new variable; controls
  the emission of logging messages
  (dbus-proxy-make-method): only produce logging messages when
  dbus-proxy-enable-logging is non-nil
  (dbus-proxy-make-interface-class): likewise
  (dbus-proxy-make-object-class): likewise