Got the collision response code implemented, although it has some pretty major issues.
As objects bounce around, they appear to gain momentum until they become so fast that
they can tunnel through the walls and disappear forever.  This even happens in an
entirely inelastic collision (set delta to -1.0f * closingVel in ObjMan::collisionResponse
to get the same effect as having a coefficient of restitution of 0).  Probably have a sign
flipped somewhere...
Changed the collision resolution to use an iterative algorithm.  This is slower, but I was
noticing some issues with the existing implementation.  It may be necessary to move back to
a non-iterative implementation later.  Also, the call to sort is entirely unnecessary, but I left it in there for now
anyways.  Its possible that by resolving the larger intersections first, we reduce the number of iterations.
It's also possible that the overhead from calling sort on every iteration is enough to negate this, but IDK.

Still have not fixed the issues with the incorrect velocity changes in the collision response.