Edit Case Activity - Update date/time when completing activity

When using the "Edit Case Activity" screen, the status may be changed. Use jQuery to observe changes in the status. If the new status is "Completed", then:

1. Set the activity_date_time to the current time
2. Display a black popup (CRM.alert) with

• Title: Updated Completion Time
• Content: Changed from "2013-01-01 02:10" to "2013-02-02 14:30". [_Undo_]
  (If user clicks "Undo", then restore the previous value of activity_date_time)