Added icons option to chart

I needed a timeline that allowed displaying timespans and your plugin fit the bill. But I also wanted the ability to show icons instead of text labels, so I added the option to your plugin. If you like it, feel free to accept this pull request. Or reject it if you don't. :-)

To use it, just add an "icon" key to each of your datapoint lists and then call "icons()" during the timeline instantiation. (Example added to examples.html)

Thanks!