Set wm class and name

Currently, dunst does not set neither provides a way to set the name and class of the notification windows.

This is makes it not possible to customize the behaviour of them on tiling window managers, (e.g. http://www.haskell.org/haskellwiki/Xmonad/Frequently_asked_questions#Make_new_windows_appear_.27below.27_rather_than_.27above.27_the_current_window).

This PR enable users to configure the class and title of the notification windows.