Convert app to modular style and include atlas_assets files

I've put the SCSS files in the /assets folder and now they can be accessed in the browser at http://localhost:5000/assets/atlas_assets.css (note that the "stylesheets" and "javascripts" folder names are skipped in the url)

Also, to run Rack::Builder (which is useful for mounting multiple Sinatra Apps to the same Rack instance) you must write the Sinatra application in the modular style, which is to say as a class inheriting from Sinatra::Base and then run an instance of the class. I've updated app.rb to reflect this.