Ragdoll Part 9: 

Adds the nearest avatar (if close enough) to the ragdoll simulation (only adding one other avatar ATM until the broadphase culling system scales better as we increase the number of shapes to the simulation).

The ragdoll positions are now stored in the "MyAvatar-relative-frame" -- the cardinal axes are parallel to those of world-frame, but the origin of the simulation is at MyAvatar's hips.  Should not be noticable to the end user, but was needed to add more ragdolls and will show up in the diffs.

The two ragdolls do interact, but the other avatar will only respond if it is doing the simulation correctly on its end.  Dunno how well this works because I wasn't able to test it much with a single computer.

Known issues:

(1) Trying to walk into another avatar will disrupt your avatar's skeleton but won't deflect the the avatar's position.  Work needs to be done here to allow avatars to push each other around.

(2) None of the problems mentioned in Part 8 have been fixed.  The priority was to get a second avatar in the system.
Needed to prune extraneous trees from the Model::_jointState data structure because they were causing problems with the collision system.  Dunno why we would ever want more than one root joint to a Model, but if we ever find a reason then we'll have to remove this code and figure out how to handle their collisions properly.