constraints: example of a motion constraint using gravity instead of friction.

No visuals, but it really nicely shows how just tweaking the manipulable can give a very different kind of interaction.

Next up:
 - rebounding constraints instead of just springs
 - snappy motion constraint (for pagers, etc) -- feels like a "modulo" constraint or something like that.
 - better, richer examples
 - maybe a parser to reduce the example sizes; can I reuse or extend GSS?