The -cancelForDelegate: method was working for downloads but not local cache checks. Add some new machinery to keep track of pending cache requests and only message the delegate if it hasn't since requested cancellation.
The helper (now called manager) is now handling the mapping between the UIImageView and its downloader.
This way we don't polute the UIImageView, and don't remove its capability to have subviews.
This change removes the automatic handling of image placeholder. The placeholder image can
be passed as second argument of setImageWithURL:placeholderImage:
The manager now handle duplicate downloads for the same URL gracefuly by sharing the same downloader
for all requestors.
Finaly, the manager handles URLs which can't create an image (HTTP error or invalid format) by flagging
them so it won't retry to download them again and again.
Olivier Poitrey
Adam Ernst
Jeppe Vesterbæk
Jamie Pinkham
Elio Gonzalez