Add drawCircle(); implement move() and render().

particle_collisions/src/particle_collisions.c
	-Implement `move()` and `render()`.

particle_collisions/(makefile, src/screen.(c, h))
	-Replace `drawPixel()` with `drawCircle()`, because the
	particles will be represented as circles rather than single
	points.
	-Implement `drawCircle()` using the `SDL_gfx` library; add the
	necessary flags to the project makefile.