Use HTML5.1 <main> tag

HTML 5.1 adds support for the <main> semantic element to identify the main content of the page. The recommendation is to replace <div role="main"> with <main role="main">. See http://www.webmonkey.com/2013/02/main-element-lands-a-starring-role-in-html/ for more details on <main> and links to references.

This set of changes implements the <main> element in the-bootstrap (based on searching all files for role="main"). A minified version of the latest html5shiv (https://github.com/aFarkas/html5shiv/tree/master/src) is included in the js directory to add support for the main tag for browsers which do not currently support it (changes do not include un-minified version). Note that, when I pulled it, main had only been added to the unminified shiv, so I minified code using an online minifier. Also note that this is html5shiv-printshiv.js.