Port to atom-shell

I started porting this to atom-shell just for fun. It is working with atom-shell >= 0.13.2 but atom-shell currently dosen't support @2x images used as a menuicon, so the icon will look fussy on a retina display.


add yourself as a `contributor` ;)

https://www.npmjs.org/doc/json.html#people-fields-author-contributors