port canvas3d javascript renderer

We need to:

 * include the `canvas3d_lib.js` file (make sure it's BSD-licensed)
 * make html messages when canvas3d objects are created which call the library (see `<div><script>canvas3d.viewer("%s");</script></div>`, where url is the url to the file (which would be `cell://filename`)

Thanks to Kevin Li on sage-devel for pointing this out.