Use BootstrapCDN in libraries configuration. Closes #1564

This PR introduces CDN version of Bootstrap both for current Bootstrap 3 version (3.1.1 as of time of writing) and latest version of Bootstrap 2 version (2.3.2) to be used in JSBin editor "Add library" feature.

The 2.3.2 version uses slightly different assets when enabling Bootstrap 2 responsive features - it uses combined stylesheet file instead of two separate files as per Bootstrap 2 docs - but that is how [bootstrapcdn.com](http://www.bootstrapcdn.com/) hosts Bootstrap versions I think.

Tested locally