Properly unlink all old avatars when deleting/uploading a new

We're also now using $config['image']['jpegquality'] to determine the
quality setting for resized images.

To set Avatar max size, adjust $config['avatar']['maxsize']

The getAvatar call now throws exceptions too. Related changes applied.
Now let's move Profile->avatarUrl to the Avatar class!

classes/Managed_DataObject.php

@@ -111,7 +111,7 @@ abstract class Managed_DataObject extends Memcached_DataObject
     }
 
     /**
-     * Get a multi-instance object in an array
+     * Get a multi-instance object separated into an array
      *
      * This is a utility method to get multiple instances with a given set of
      * values for a specific key column. Usually used for the primary key when