Accordion: Add the ability of populate accordion with JSON.

I find this is a useful feature when we want to dynamically populate the
accordion with JSON return by things like Ajax calls (instead of constructing the HTML on the fly). It is also easier to visualize the structure. I also added a few demos to show it. Basically, I added a 'data' option and it defaults to null. Meaning that this ability is disabled when data is null. When data is a well formatted JSON, it will populate accordion with JSON data. I also have more ideas that I would like to implement on top of that.