Add Sass support

Projects will be generated with ember-cli-compass-compiler, providing support for Sass and Compass. The app/styles/app.css is renamed to app/styles/client.scss (client.scss because it compiles to client.css, which is referenced in the index.html file). Any other Sass files in app/styles will be compiled, but you should (obviously) import them into client.scss instead of importing their css output directly.