changed from growl_notify to libnotify

growl_notify was causing an error while installing rb-appscript (0.6.1) (OSX install).  Followed the suggestion in,
http://stackoverflow.com/questions/9660919/error-while-installing-rb-appscript-gem

to switch to libnotify.  I'm assuming here that we still want to pass the same argument to libnotify as growl_notify...