Use $.getJSON for more reliable JSON parsing

Hi, this looks really nice! I was getting errors loading the JSON file in Firefox though, which was because `$.ajax` was parsing the json file before calling the success callback, and `JSON.parse(<array>)` apparently doesn't work.

Also, one can use [GitHub Pages](http://pages.github.com/) to host a static version of the webpage. All you need to do is create a branch called `gh-pages` with the content desired, and then you can access it [like so](http://huonw.github.io/burning-down/map.html). (I just created the gh-pages branch directly on top of my `getJSON` branch for that link.)