FEAT #11 Rope joint support added

Thanks to Aaron Farr, I now have a version of box2dWeb.js that supports rope joints, which are kinda crucial for implementing a grappling hook gun. Why don't I just call it a harpoon?
FEAT #11 Grappling gun fires and attaches

Rope behavior is working okay, but I'm lost as to how to draw the rope. The gun has a tendency to fly out of the astronaut's grip, especially when the prismatic latch joint is active. Need a more robust way to have astronaut "hold" an object firmly. Also, apparently world.SetContactListener() destroys any prior listeners, so I'll have to glom everything together in one scene-level contact listener.