Add axes, axis labels, and a legend.

[chrisranderson]

Thanks to @wilcoschoneveld (see #41) for this.

I explain a lot of this in the README, but it would be better as part of the visualization, and make it easier to count and trace stuff back to your code (e.g. Huh, that filter looks interesting, but there are 256 of them, and I'm not going to count all of them to figure out which one it actually is).

I'm not sure how all this would work, given that it's just one big image I'm sending across.