Support for turtle tests

Much of this is taken from brandon's #151.  As noted there, this results in a new native dependency.  It would probably be worth having someone on a Mac make sure they can get this working.  The comments at #151 contain a link to instructions.

jsdom + the node-canvas module seems to have a bug wherein it loses the id of canvas objects.  This is why we moved away from getting the canvases via getElementById.

I've added tests for just 2 turtle levels to start, to prove things worked.  The next step for me is probably to start improving coverage for our levels - for both success and failure cases.