Replace grunt with brunch to build the app

- No more huge Gruntfile.js
- Using brunch conventions (app/ subdir)
- Use native sass lib (no more ruby compass, which means no more Gemfile)
- Simpler index.html
- Damn faster build

To Do:
- Minify js/css/html/images
- File revisions
- Bring back livereload
- Production overrides
- Bring back qunit
- Bring back jshint
Remove .ruby-version file
Don't install Bundler and Grunt, install Brunch in the bootstrap script
Improve last instruction of bootstrap script