Thumbs: center within container

cause why not?
Thumbs: improve styling a bit

* round the corners more
* use % sizes so they look good at any side-bar width
Responsive: make the layout a bit responsive

On narrow screens put the text below the images. Not sure if
this is the best idea... but hey, responsive!
Responsive Images: do it for reals!

Now, which image url is used can be adjusted after DOM creation. If
utils.adjustResponsiveImage(img), a new src url will be chosed based on the
current width of the image. Also, we memoize the last recorded image size
so that upon creation of a new one of the same type we can start it out at the
same size as other images. This prevents double-downloading if the initialWidth
is small and the current width is large.
Responsive Images: re-adjust on widnow resize

With a throttling of course.