Implementation of TOC for articles (blog, news, documentation).

Attempt to address #87. Based on:

- https://github.com/ghiculescu/jekyll-table-of-contents
- http://stackoverflow.com/questions/7985081/how-to-deploy-a-jekyll-site-locally-with-css-js-and-background-images-included

Several options available to customize toc() behaviour. In particular we may want to add a yaml option to disable TOC on some pages using the article layout...

In addition we may add a CSS...