Working-range-driven image loading.

Add a `usleep(1.0 * USEC_PER_SEC)` delay to ASBasicImageDownloader and
slowly scroll through the Kittens sample project.  Without this patch,
you'll see that images only start downloading after their purple
placeholders appear onscreen.  With it, images can download and render
before you scroll them onscreen, thanks to the working range.
Merge pull request #130 from facebook/range-driver

Working-range-driven image loading.