Bootstrap 2.3 assetic

This PR aims to add bootstrap 2.3 asset to symfony:

Reference:
https://coderwall.com/p/kzyiaw
to add bootstrap:
- change the composer.json
- > `../composer.phar update`
- change the file config.yml
- > `php app/console assetic:dump --force`
- > `php app/console cache:clear`