added ability to search by author display name

Here's an implementation for allowing users to search by display name.
Wordpress doesn't have a great way of hooking into a display name
change, so I had to hook into edit_user_profile_update and
personal_options_update then retireve the POST var from the form. This
isn't a great way of doing things, but since those 2 hooks fire before
the author is updated it's the only way to get the new display name.