Keep a reference to the image URL
Add documentation to imageURL property
Add imageURL property to remaining categories
Scratch that, this won't work on UIButton
Store image URLs by state in the UIButton category
Conform to coding style
Merge branch 'imageURL-property' of https://github.com/klaaspieter/SDWebImage

Conflicts:
	SDWebImage/MKAnnotationView+WebCache.m
	SDWebImage/UIButton+WebCache.m
	SDWebImage/UIImageView+WebCache.m