Category methods collision

Hey, there was an interesting issue on RestKit project https://github.com/RestKit/RestKit/issues/1285

Looks like AFNetworking UIImageView category shares some method names with that of SDWebImage, which leads to weird bugs as seen above. 

https://github.com/rs/SDWebImage/blob/master/SDWebImage/UIImageView%2BWebCache.h#L53
https://github.com/AFNetworking/AFNetworking/blob/master/AFNetworking/UIImageView%2BAFNetworking.h#L43

I wonder if there should be a af_ prefix, I understand that this will lead to big backwards compatibility breakage :/

/cc @rs