Feature/grunt

Integrate grunt into the RadEdtion. We use css compass by default, but i am open to advise. It's quitte simple to use. First a simple `npm install` for all grunt dependencies is needed. Second, three commands are available on grunt. To start developping use `grunt serve` that install the front app (located on `src/App/Resources/front` into the web directory (`web/front`). Bower dependencies are automatically installed into the `src/App/Resources/views/layout.dev.html.twig`. Once the develop is finish just launch `grunt build` to build a minified application with a given `src/App/Resources/views/layout.prod.html.twig`. 

Hope you like it ;)