Use therubyracer since node isn't always around.
Catch 404 etc when image is unavailable.

Just returning means we display a blank image due to the code in
app/views/. A 404 probably means the image is gone for good. Perhaps a
better strategy would be to clear out the .image property.

We could then have a separate populate task to fetch missing images.
Directory for cached images needs to exist.
Add some install and todo notes.