Now uses 'natural box model' for all of site.

The entire site (except for a few thingies in `gallery.scss`) now uses
the 'natural box model' advocated by Paul Irish (among others).

    html { box-sizing: border-box; }
    *, *:before, *:after { box-sizing: inherit; }

See also: http://www.paulirish.com/2012/box-sizing-border-box-ftw/