working out the extra positions logic.  but it got ugly.  about to change the plumbing.
changed the plumbing to put the extra state on the end of the state vector (instead of in the middle as a fake position)
lots of changes in here.  including a change to the inputs of manipulatorDynamics.  (but one that is easy to catch)
xx must be an optional argument (manipulators with no extra state need not implement it)
swaps control surface to explicity compute affine terms instead of using a differen chord length and using the parents computation.
Merge branch 'controlsurface' of github.com:RussTedrake/drake into controlsurface