Add mse (mean squared error) and psnr (peak signal to noise ratio).

Although, this may not be the final solution I think we can merge it.
Each cell in the table now has fields the show the mse and psnr
of the image as compared to a reference image. The reference
images were created using cairo, except for the gradient images
which used rasterific. Hence when comparing cairo you will see a
mse of 0 and psnr of infinity.

For SVG and Postscript, ImageMagick is used to convert SVG and EPS
to PNG. 

* ImageMagick is unable to convert SVG gradients.
* I still don't know what to do about postscript since all of
  our reference images have transparent backgrounds and
  postscript has no alpha. I'll probably try to find some way to
  replace a transparent layer, with a white layer in the reference
  images for camparison.