Make it easy to save videos of demos

[keenon]

This is totally not necessary, but would be nice. I'd like to be able to call a method from Python, and have it save a video that was rendered on the web GUI. Possibly even on a headless browser, if for example you're doing experiments on a beefy AWS machine and want to generate videos async. Here's an interesting proof of concept: http://www.smartjava.org/content/capture-canvas-and-webgl-output-video-using-websockets/