DOC optimized PNG images with OptiPNG

Did "optipng -o 10" on all the files with OptiPNG 0.6.4.
This shaves a total of 529012 bytes off the pictures.

Conflicts:
	doc/testimonials/images/rangespan.png
	doc/testimonials/images/yhat.png
	doc/themes/scikit-learn/static/img/FNRS-logo.png

(Testimonials not on this branch, so I just deleted those images.)