…loads

At the moment the ASBasicImageDownloader does not automatically retry image downloads if the remote
host is unreachable. On the contrary the ASPINRemoteImageDownloader automatically retries. Retrying is
something that ultimately clients need to be able to control, for example to fail fast to an alternative image
rather than keep retrying for more than one minute while not displaying any image. This change exposes
a new option in the ASImageDownloaderProtocol to retry image downloads. It also uses this new option
in both ASNetworkImageNode and also ASMultiplexImageNode, setting it to YES to preserve the current
behaviour.

ScreenNode.m is implementing ASImageDownloaderProtocol and needs to
be fixed to reflect changes in the latter.