Icon and a small fix

First things first, thank you so much for this app. It has been an enormous help to me. 

I use it enough that I put it in my Dock, and it started to irritate me that there wasn't an icon, so I created one. I don't know if you care for it, but I thought I would send a pull request anyway. If you want it, I can also include the original Inkscape .svg file for the icon.

I also made the small change to have it quit when the last window closes.