Add horizontal navigation bar to page bottom. 

This will add a horizontal navigation bar to the bottom of all documentation pages, with a previous- and next-link. It has been tested in IE8, IE9, Safari 6 (Mac), Chrome 26 (Mac), and Firefox 20 (Mac).

A screenshot of this implementation is [here](https://code.djangoproject.com/attachment/ticket/16357/%20django-docs-bottom-nav-mockup-02.png).