Enabling sourcemaps on yeoman generated gruntfile

Still not perfect, but goal is to demonstrate that it is possible to :
- Have sourcemap (needs sass pre-release installed : `gem install --pre-release sass`) enabled on your project
- Livereload on sass files in chrome

Not perfect though :
- There are a lot of limitations (see my comments)
- Chrome's Workspace feature doesn't seem to love .tmp/styles/ folder (dotted directories ??)