Update README example

Current example uses `var backburner = Backburner.new( ...` which does not run because the constructor is actually available as  `window.backburner.Backburner`. Rework the example to fix this. Also added the surrounding HTML to make running the example just a copy & paste operation which is nicer for newbies I think. Happy to revert the HTML stuff if you would prefer - just let me know.