Make more pages validate, this should be most of them.
Fix tag in license.md, see below.

- Jekyll was optimizing <span class="current-year"></span> to
  <span class="current-year" /> which is invalid.
- Because it was invalid, browsers (and the validator) were treating it
  as a <span> start tag. Because of this, when the text of the span was
  set via javascript, some of the copyright info was being lost.

So instead of
"Copyright (c) 2002-2013 EPFL, Lausanne, unless otherwise specified. \
All rights reserved."

We were seeing just:

"Copyright (c) 2002-2013"
Make hacker-guide valid.
Add a small script to check validation of _site/