Changed NPM version badge to use shields.io badge.
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.
Added change log to the readme file.
Added .npmignore to the .npmignore file.
Bumped version to 2.1.0.