[ODE] Joint groups

[Michal Bacik]

It seems that joint group is suitable only for one thing: fast allocation of
contact joints, and fast clean-up after stepping the world.

I wonder if there's some other usage. If not, it'd be cleaner not exposing
'another interface' to user, and managing the group internally.
dJointCreateContact would add new joint to the group, dWorldStep would call
dJointGroupEmpty.

Of course, if there's no other reason - examples doesn't imply that.

- mb