First cut at sit entity script

* supports all entities with sitting points

examples:
* https://s3-us-west-1.amazonaws.com/highfidelity-public/ozan/CinemaChairs.fst
* http://highfidelity-public.s3-us-west-1.amazonaws.com/ozan/theater.fst
* http://public.highfidelity.io/models/content/sittable_chair.fst