Implement If-Modified-Since when downloading images

[inercia]

Our HTTP client currently downloads the image specified on source even when the image in the server is the same as a local copy previously downloaded.

For ths case where a local copy is already in the pool, we could perform the HTTP request with the If-Modified-Since header (using the modification time of the local image), so it would be downloaded only if the remote file has been updated.

[MalloZup]

@inercia good idea