Fix #4: Avoid undefined force values.

If a spring is infinitely compressed, the direction
of the force is undefined. This results in NaN values
unless specifically set to zero.
Allow "throwing" objects with the mouse.

When objects are dragged and released while the simulation
is running, they will now keep the speed at which they
were dragged during the most recent timestep.

While they are still pressed, their speed will remain zero.

This adds a new simple example, in which a pendulum
can be "thrown" into a stable elliptical orbit.

(resolves #2)