Implement SSIM as a metric.

[riaanvddool]

SSIM would be easy to implement in python [1].
The gradient can be calculated [2] so we should be able to use this as a metric?

[1] http://en.wikipedia.org/wiki/Structural_similarity

[2] Zhou Wang and Eero P. Simoncelli. Maximum differ-
entiation (MAD) competition: A methodology for com-
paring computational models of perceptual quantities.
Journal of Vision, 8:1�13, 2008.

[nfaggian]

I like the idea, if you could look into this it would be really great.