Rename id selector of comments to avoid clash with generated IDs from markdown.
Move mathjax to the bottom of the page to avoid the browser stalling.

This was accomplished by changing the `mathjax.pm` script/plugin from adding
the script right before the HTML tag `</head>` to right before `</body>`.

Note that Google's suggestion of putting it below `</html>` makes the w3
validator (and Firefox's parser) complain.
Change ID of comments to avoid collisions.