Enable Juvia as an alternative to Disqus comments.

This patch adds support for the open-source Juvia comment system (https://github.com/phusion/juvia) as an alternative to Disqus.

It requires two extra parameters in _config.yml: `juvia_site_key` and `juvia_host`. I added a pull request on Octopress which adds Juvia support to the original theme (and the _config.yml parameters too): https://github.com/imathis/octopress/pull/1224  Since I use whitespace on my own site, I've added support to this theme too.