Add visualizations/prime_spirals dirs, READMEs.

visualizations/README.md
	-Add top-level README for all visualization projects.

visualizations/prime_spirals/README.md
	-Add a README for the prime-number spirals projects.
Add HTML page to display prime-spirals.

visualizations/prime_spirals/index.html
	-Add a boilerplate HTML document to display the canvas-rendered
	prime-number spirals.
Add graphics/spirals modules; define setupCanvas().

visualizations/prime_spirals/graphics.js
	-Add module to handle all canvas graphics.
	-Define `setupCanvas()`, which exports `drawPixel()`.

visualizations/prime_spirals/spirals.js
	-Add a placeholder spiral-rendering module.
Define ulamSpiral() in spirals.js.

visualizations/prime_spirals/spirals.js
	-Define a completed and functional `ulamSpiral()`.
	-Add documentation comment.
Define sacksSpiral() in spirals.js.

visualizations/prime_spirals/spirals.js
	-Define a completed and functional `sacksSpiral()`.
	-Add documentation comment.
Document ulamSpiral() -> drawLine().

visualizations/prime_spirals/spirals.js
	-Add a documentation comment to `drawLine()`.
Move javascript files to dedicated dir js/.

visualizations/prime_spirals(/ -> js/)*.js
	-Move all javascript files to their own directory for greater
	modularity and less top-level clutter.
Add primality lib submodule. Fix index.js links.

.gitmodules, visualizations/prime_spirals/js/primality
	-Add the primality lib, used by `spirals.js` functions to check
	the primality of values.

visualizations/prime_spirals/index.html
	-Fix `script` `src` values, which weren't updated to reflect
	2695aae.
Add Sacks/Ulam spiral images; content to README.

visualizations/prime_spirals/README.md
	-Add explanation/sample usage of the functions exported by
	`js/script.js`.
	-Add images of either spiral (contained in `img/`).

visualizations/prime_spirals/img/(ulam_spiral, sacks_spiral).png
	-Add images of the Ulam and Sacks spiral, for use in
	`README.md`.
Add header comments to all JS files.

visualizations/prime_spirals/js/(graphics, spirals).js
	-Add header comments documenting each file's contents.
Merge branch 'visualizations/prime_spirals' into develop

visualizations/prime_spirals/
	-Add a prime number (Ulam/Sacks) spirals visualization.