Change license to LibreJS

[sinak]

From folks at FSF:

I imagine that the widget will also include JavaScript, and I was hoping
that you'd also look into tagging it so that it can be easily identified
as free software and recognized by automated systems, like GNU LibreJS
(https://addons.mozilla.org/en-US/firefox/addon/librejs/).

This is currently the gold standard for free JavaScript, which the FSF
is helping promote. We've got a page up on GNU.org to help people take
the very simple steps necessary to make their code LibreJS-compliant
(http://www.gnu.org/software/librejs/free-your-javascript.html), and I'd
be glad to answer any questions you have about it. If you'd like more
info, you can also check out our Free JavaScript campaign:
http://www.fsf.org/campaigns/freejs

[sinak]

Also, let Zak @ FSF know when this is done.