Added jQuery plugin and $.support.boxSizing property.

The jQuery plugin applies the boxsizing.htc behavior asynchronyously as
to not delay the rest of the page. If the browser already support
boxSizing, the plugin does nothing.

Usage would be something like:

$(document).ready(function(){
    $.fn.boxSizing.defaults.htcPath = '/js/boxsizing.htc';
    $('*').boxSizing();
});