Auto-turn body when head yaw is beyond 45 degrees in HMD

In headmove.js, detect when the head is turned beyond 45 degrees in either direction, and begin turning the body in a series of step turns (so as not to be nauseating).  

I set the steps and stepping period fairly small, because I think this is a nice tradeoff between getting closer to a sensation of continuous turning and not inducing nausea.  See what you think.