Rollingdisc example

See for 2a13a1a comments. This isn't ready to be merged; I'm just opening the pull request so I can ask questions.

One more comment is that I think I'm accessing states the wrong way when setting the camera's location/orientation. See https://github.com/chrisdembia/simbody/blob/rollingdisc/Simbody/examples/ExampleRollingDisc.cpp#L46; is there a better way to get the x and z position of the contact point?