Update Bootstrap and use bootwatch themes hosted by bootstrapcdn.com

Easy support for multiple bootstrap themes designed by bootwatch and hosted by bootstrapcdn.com.

I added bootwatch variable to the JB hash in _config.yml so the user could add the theme URL. And added a conditional to the head of the default.html to load the theme.

For consistency, I also used a CDN for the bootstrap v2.3.1 css and js hosted by bootstrapcdn.com. And pointed the jQuery googleapis CDN.