Migration to rails 3.1

#### Changes made:
 - edited Gemfile
 - moved files from public/ to assets/
 - added `config.assets.enable = true` in config/application.rb
 - commented lines about rjs in config/environments/development.rb
 - added lines about uglifier and scss
 - added app/assets/stylesheets/application.css
 - added app/assets/application.js

#### TODO:
 - move application.js in app/assets/javascripts/
 - resolve images not found
 - remove files not needed like rails.js, jquery.js...