Test turtle

This is an extension of #150 to support Turtle testing. Please merge that first.

## Warning: New Native Dependency!

Since Turtle uses Canvas, I needed to take a dependency on [node-canvas](https://github.com/LearnBoost/node-canvas) which has some manual installation steps: https://github.com/LearnBoost/node-canvas/wiki/Installation---OSX

I'll send a PR to automate these steps for Ubuntu under Vagrant, but that won't help OSX users. Let's wait until after the current deadline to re-evaluate when to merge this PR and how to handle the dependency. Just publishing this PR, so that the code is backed up and visible for review.