Initial commit.
Add obstacles in phystest with the the mouse.
Face sides the other direction.

Before, drawing a polygon in the clockwise direction would make all of
its sides pointing inward, now they point outward.
Mark mouse events as changing the scene.
Put keyboard events in their own functions.
Simplify movement.
Make the default sides bound the window.
Add d key to delete sides.
Fix poor comments.
Add gravity.
Remove color variables.
Only set stopped=true when velocity is zero.
Add gravity directly to velocity.

The paper [1] recommended doing it in two steps, but this seems to
have a problem: when colliding with things above the circle, gravity
moves down by two pixels and it doesn't look like the collision was
flush. The problem that the paper sites with adding gravity directly
to velocity doesn't seem to matter.

[1] http://www.paulnettle.com/pub/FluidStudios/CollisionDetection/Fluid_Studios_Generic_Collision_Detection_for_Games_Using_Ellipsoids.pdf
Rename Side to Segment.

The name Segment is more clear because these are line segments.
Make upward velocity counteract gravity.
Add acceleration for gravity.
Add a function to round float64s to ints.
Add Ellipsoid and Ellipse.
Add Point.Times and Vector.Times.
Add ellipse-based collision detection.