Migrate to bootstrap version 3

This proposes migrating explanations of `2012-04-18-app.markdown` to bootstrap version 3.

Note that this migration cannot be completed with only merging this pull-request.
It needs to deploy the following files into `assets` directory on `railsgirls.com` server:

- http://netdna.bootstrapcdn.com/bootstrap/3.0.3/css/bootstrap.css
- http://netdna.bootstrapcdn.com/bootstrap/3.0.3/css/bootstrap-theme.css
- http://netdna.bootstrapcdn.com/bootstrap/3.0.3/js/bootstrap.js