Added a camera class with tests.
Part of the world physics simulator implemented.
Removed unused module material.py.
More physics.
Removed unneeded file.
Some fixes to the physics simulator.
Added a rendering engine plus some testing code.
Physics fixes.
Disk.isInContact now uses vector operations.
Some collision detection fixes, plus more debugging code.
Coordinate conversion in Renderer. More debugging code.

The renderer class now has two methods for coordinates conversion:
surfaceToWorldCoords converts surface coordinates to world
coordinates, and worldToSurfaceCoords converts world coordinates to
surface coordinates.

Some debugging code was added so that we can manipulate the ball by
the mouse, pause/unpaused the simulation and advance simulation
step-by-step.
Disk-specific collisions.

Now instead of one Collision object for two disks, two Collision
objects are created, each specific to one of the disks.
More debugging code.

Collision detection code can now be examined interatively.
Zooming capability in the camera. User can zoom with mouse wheel.
Camera panning implemented.
Keyboard changes. Dragging velocity fix.

ESC key now cancels dragging. Arrow keys don't change speed anymore.

Dragging now causes less velocity.
Anti-aliased disks.

Anti-aliased disks are now drawn using pygame.gfxdraw module.
Fixed time-step for physics.