atom-shell child process

Sets up desktop app packaging for mapbox-studio as a child process of atom-shell. node-based execution is still possible/supported.

- [x] @yhahn use travis to do mac os x .app signing
- [x] custom Info.plist for OS X app
- [x] @camilleanne Mac OS X menu
- [x] @camilleanne handle downloads/external links in atom-shell, blocked by https://github.com/atom/atom-shell/issues/542
- [x] **PUNT** ~~review windows uninstaller~~ https://github.com/mapbox/mapbox-studio/issues/611