Changed runNextUri() calls to use setImmediate().
Changed runNextUri() calls to use setImmediate().

The call stack was becoming alarmingly large when there were long lists
of image URLs to process. This commit pulls runNextUri() calls out of
the current call stack.

sunpech/interfacelift-downloader,ashking/interfacelift-downloader,stevenbenner/interfacelift-downloader