This adds native OS X user notifications as fallback, if Growl is
currently not running. Those native notifications are only available
on 10.8+, therefore some preprocessor conditionals prevent that this
breaks 10.7 compilation.
There is no explicit check if the class is available as it should be
on all 10.8 systems.
Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>