Navigation enhancement

If you're working on a project with a large number of partials, the single column navigation can quickly disappear off the bottom of the page.

To solve this problem I have added a new Handlebars helper {{#grouped_each}} to helpers.js which loops around every nth elements.

I updated the template.hbs file to use this helper when generating the navigation so that every 20 navigation elements a new list is created.

I have updated the theme.css to adjust the navigation layout.