[WIP] Migration to rails 3.1

In reference to #575.

#### 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/javascripts/application.js
 - removed some unnecessary JS files (jquery.js, rails.js, prototype.js) to use the gems

#### TODO:
 - resolve some images not found
 - remove problems in tests