update sample to work with firefox (no auto instanciated id)

[enguerran]

Despite the "spec", firefox cannot handle the id-to-global-variable in the demo.html.

I found some resources speaking about this spec I did not know:

• https://stackoverflow.com/questions/6381425/is-there-a-spec-that-the-id-of-elements-should-be-made-global-variable
• http://www.2ality.com/2012/08/ids-are-global.html
• https://developer.mozilla.org/en-US/docs/Quirks_Mode_and_Standards_Mode

What do you think?

[addyosmani]

I'm all for improving overall x-browser compat, but wasn't aware FF wasn't able to handle this properly. I'll do some testing against master and your PR to verify this is the best fix we can opt for. Thanks again (and sorry for the delay reviewing!)

[enguerran]

No problem for the delay, give me some feedback when you can. We all know you can be a very busy man.