Slight project-wide cleanup, refactoring.

Note: virtually every file was modified, but in minute ways that don't affect
functionality. See diff for the gory details.

Mod:
	src/screen.c
		-drawPixel() no longer has a color parameter, simply defaulting to
		TEST_COLOR.

Mod, Ref, Rea,:
	src/**/*.c
		-add file-header/function-header documentation comments to any
		files/functions that didn't have them.
		-remove superfluous include statements
		-increase modularity by declaring unnecessarily public functions as
		static.