Replace life curtain with simple cell count

In order to prove that a scene can run a simple simulation -- in this
case Conway's Game of Life -- adding a curtain containing a div for
every cell was the path of least resistance. This, however, proved to
create a serious limitation in rendering time. My hypothesis is that it
was the large overhead of manipulating DOM elements (over 600 of them)
that caused the slowdown, and not anything inherent to the simulation or
implementation of scenes and actors.

Half of that has already been borne out by removing the complex curtain
DOM elements and replacing it with a simple count of live cells. The
full simulation is still running and appears to be maintaining a very
high framerate, as we should expect.

The next test is to see whether a canvas-based visualization will be
more efficient, or whether rendering 600 to 1000 of anything is simply
prohibitive in the browser.
Add stage with Paper.js

Proof that my hypothesis was incorrect. It is indeed my naive
implementation of the Game of Life that is causing the slowdown. This is
probably good news. It means that drawing ~2000 shapes to the canvas
isn't inherently prohibitive.

As for the addition of the stage, this is a rudimentary implementation.
The stage is assigned right in the game js without leveraging anything
from src/stage.js. All it really does is initialize a set of symbol
instances for paper (one dot for each cell) and define an evaluate()
function that will loop through the dots and update their visibility to
match the associated cell's alive state.
Use stage to render demo

Rendering Conway's Game of Life at respectable framerates using Paper.js
to draw to the canvas during the game loop.