Fixed bug where images that had been updated on the server would not be downloaded.

Hi Jake,
I encountered a bug using your lovely lib where once an image had been downloaded it would be cached and never re-downloaded even if the image has been modified on the server.

I now send a request for the image with an http 304 if-unmodified-since with the date of the last successful download of the image.

Then the image gets downloaded only if it has been modified on the server.