Define a character encoding in the HTML.

On [slide 18](http://afabbro.github.io/jsconf2013/#18), apostrophes look weird. I think this is because they're the funky unicode kind, and I guess github doesn't deliver charset information in the HTTP headers, and there's no `<meta charset>` tag defined, so the browser just displays them as weird stuff. I tried adding the meta tag and things look okay [on my branch](http://toolness.github.io/jsconf2013/#18).