Add skeleton layout to particle_collisions.*.

particle_collisions/makefile
particle_collisions/particle_collisions
particle_collisions/src/particle_collisions.(c, h)
	-Add skeleton function declarations, like `simulate()`,
	`move()`, and `render()`.
Add screen module (a per-pixel SDL wrapper).

particle_collisions/(makefile, src/screen.(c, h))
	-Add a `screen` module, which wraps the `SDL` graphics library
	to provide simple, per-pixel access.
	-Add the necessary `SDL` library links and flags to the project
	makefile.