Move helpers includes into helpers.rb.

As recommended by nano docs (http://nanoc.ws/docs/basics/#layouts).
Update default layout to match Foundation's.

See http://foundation.zurb.com/docs/css.html,
§HTML Page Markup.
Update warning about NANOC_ENV.
Add Lifepillar's favicon.
Add humans.txt and robots.txt.
Add 404.html.
Include favicon in default layout.
Improve rules.
Ignore output.diff.
Add partials for GA, Disqus, and MathJax.