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 situation makes it not possible to customize the behaviour of the windows on tiling window managers, (See http://www.haskell.org/haskellwiki/Xmonad/Frequently_asked_questions#Make_new_windows_appear_.27below.27_rather_than_.27above.27_the_current_window and http://xmonad.org/xmonad-docs/xmonad/XMonad-ManageHook.html#v:composeAll).

This PR enable users to configure the class and title of the notification windows.

Possibly related to #68 but not really a fix for that, but can mitigate the issue.