Add skinned mesh test

[Izzimach]

As shown in #20, the skinned mesh was not kept up to date. Add a test that uses a skinned mesh. Might also make a good demo.

[poshaughnessy]

This would be very cool!

I started attempting to make a demo myself - in my Slides repo under src/components/demos/skinned/ and used in the slide src/components/slides/skinned.js which I've currently put first.

I was looking at replicating this example (source) just as a start - using the JSON model there. Although ideally for my talk I'd like to animate something more like the the robot from my demo or the Three.js monster.

I suspect I'm going to struggle though, to figure out how to replicate the work of the AnimationHandler. So if you were able to come up with a test I could use as a starting point, that would be ace!