feat(app): use checkboxes for module selection

Instead of asking for each module separately, you now get a nice checkbox dialog
where you can tick each of the modules on and off.

I didn't include a checkbox for bootstrap, because the CSS/SASS switch depends on the prior choice.

This is what it looks like:

![screenshot from 2013-07-17 17 11 22](https://f.cloud.github.com/assets/9906/812677/d70f8586-eef3-11e2-8645-0ecfebd25822.png)