compare gfx images to screenshot of entire page

This change uses casperjs to take a screenshot of the entire page for comparison with the reference image when running gfx tests, so we can add gfx tests for screens that involve native elements.

Whoops, no, that's a remnant from the previous version. Fixed in 30a34b8.

As for printing the URL, we do that above, inside the evaluated function, on the line that reads:

                                    console.log(got.canvas.toDataURL());