Add the script to do header diffs

This is the script as used for the Qt 5.1.0 release.

Change-Id: I06d0bcbd2db876bee36118a4f2fd8fb6c1033b4e
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>