Allow subclasses to override cache key and background tag for cell background images, since we allow them to override imageForFirst:last:highlighted:drawDivider:. This way the caching facility of imageForFirst:last:highlighted:drawDivider: can be used by subclasses that override that method but call super.
Merge pull request #528 from darrenw/CellBackground

Allow NICellbackground subclasses to override cache key and background tag for cell backgrounds.