Auto download for retina images

I've boosted the UIImageView+WebCache.

using setOptimizedImage instead of setImage with change the URL for the @2x version.
this way you only have to store the regular URL in your code or DB and the program will make the switch automatically

If you think we can improve or extend the feature, I'll work on it but I guess it will be pretty useful for anyone in need of DB driven image library