Add support for libnotify notifications

I added in libnotify on top of the growl notification support. Tested and working on linux, you may want to test on OSX to ensure there are no issues (there shouldn't be :).

IIRC libnotify is installed by default on all gnome-based linux distros and probably a number of others, but it wasn't installed on Kubuntu for me, and probably not on other KDE-based distros. It can be installed with libnotify-bin package: ```sudo apt-get install libnotify-bin```