add a "package" command to create binary distributions

typing this:

```
$ gunfire package
```

would generate in dist/ the packages for OSX, Windows and Linux using nw.js (node-webkit)
http://nwjs.io/

tutorial: 
http://strongloop.com/strongblog/creating-desktop-applications-with-node-webkit/

details for packaging: 
https://github.com/nwjs/nw.js/wiki/How-to-package-and-distribute-your-apps

demo projects: 
https://github.com/zcbenz/nw-sample-apps
