Discard all image scales that are more than a day older than the context.

This is an alternative to the merged pull request #3.

When we encounter an outdated image scale, we go through all scales and discard the scales that are more than a day older than the context.  That should keep old scales working that are still referenced in cached html pages.

While testing this, I found that the `AnnotationStorage` call inherits from `UserDict` which has a `clear` method to remove all items.  Because we override `__delitem__` this does not work.  So I included a commit that adds a `clear` method that does work.