Switching to kramdown

 Jekyll uses Redcarpet by default for translating Markdown to HTML, but Redcarpet doesn't handle Markdown in HTML blocks (like div's).  This PR switches to Kramdown, which *does* handle md-in-html.  However, Kramdown translates single and double quotes into curly quotes by default, which confuses downstream tools, so we have to turn that off. While we're there, we also tell it to output character entities as character entities. It also inserts the attribute:

    markdown="1"

in certain <div> blocks in Markdown files to turn on md-in-html processing, and regularizes the section headings in those blocks.