Load a random image into the narrow page sidebar

@adamlewinter I've got a first pass at the sidebar images up on the dev site. Representative page:

http://gadomski.github.io/glac.io/locations/hubbard/overview.html

Right now I'm just loading one of the sidebar images files in at random, though I've got some machinery in place to expand the functionality a bit, e.g. by saying "only load this subset of images in for this given page".

Thoughts?