Migration to rails 3.1

Done :
 - edited Gemfile
 - moved 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 javascripts
 - resolve images not found
 - remove files not needed like rails.js, jquery.js...
update to rails 3.1.12
resolved sql problem
fix application.js

- moved application.js inside assets/javascripts
- removed jquery.js, prototype.js and rails.js to use the gems.
fix problem with ImageAnnotation
fix deprecation warnings
fix some errors

in
 - annotation_categories_controller_test.rb
 - assignments_controller_test.rb
fix flexible_criteria problems

added some fixtures files
fix some errors in notes_controller and grade_entry_forms_controller
fix some errors in tests

for:
 - results_controller_test
 - rubrics_controller_test
Merge branch 'master' into pr/1153
Merge branch 'master' into pr/1153
mysql2 >= 0.3 required in gemfile as of Rails 3.1
Fix exception type mismatch in create_or_update_from_csv_row in rubric_criterion.rb
Fixed functional test cases for move_criterion
Flash now requires has? rather than has_key?
Fixed exception in grade_entry_form.rb, fixed failing tests in grade_entry_forms_controller_test.rb
Fix issues with sessions. Moved prototype-snippet.js and rails.js to
assets/javascripts, re-enabled no-conflict
Fix failing test in test/functional/marks_graders_controller_test.rb
update_files in submission_controller now casts file_revision values to int. This fixed the failing tests in submission_controller