Add support for replacing objects upon saving to AdminObjectEdit.

This is useful for things like SiteImage where replacing the id of the object is desirable to circumvent browser and CDN caching.
Fix a typo in the comments.
Better $old_object cloning and deleting.

 * Only clone the $old_object when it's needed.
 * Only delete the old object on edits.
 * Fix a long standing issue where the $old_object is added to the cache clearing queue after it has been updated. New code correctly clones it before it's values are modified.
Merge pull request #27 from isaacgrant/admin-edit-object-replace-object

Add support for replacing objects upon saving to AdminObjectEdit.