add option to disable tray icon

[doctorcolossus]

Currently screenrotator displays an empty tray icon with only one context menu entry: 'quit'. I was initially confused as to which application this icon even belonged to. I have no use for it, as I would just kill screenrotator with pkill if I ever needed to. I never need to anyway, since I launch it in xinitrc and personally want it running the whole time I'm in X anyway. So a configuration option to disable this icon would be nice. :)