When removing 'no-js', also add a 'js' class.

Adding the 'js' class is demo'd in the originator of the no-js
thing at http://www.paulirish.com/2009/avoiding-the-fouc-v3/
and commonly used in other places that do the no-js dance. It
can come in handy.
Merge pull request #659 from jrochkind/no_js_add_js

When removing 'no-js', also add a 'js' class.