Added a javascript slideshow of images in skimage.data

To address #989

I am not used to HTML/javascript, so any HTML/javascript/CSS advice will be appreciated.
If everyone agrees with the approach, I will go ahead and document parts of my code.

The javascript slideshow [Galleria](http://galleria.io/) was chosen because it was simple to use and allowed lazy loading of images.

I have put things as I see fit, but I won't be surprised if this seems totally crazy to others and is rejected.

@sharky93