fix IE8 mega-crash.

The root of the problem, after an hour or two of head-banging, was:
<label> ... </span>
OK, I made a mistake, but don't crash the entire browser!

This also applies the fix from http://www.nickyeoman.com/blog/html/118-html5-tags-in-ie8 to get the new <section> tag in IE8.
question edit: Call Numbas.tryInit *after* defining Question()

I'm fairly sure this fixes #117, finally.