Add script to help merging

This script has to be run from a directory with the modules checked out.
By default it merges stable->dev but can be used for any direction.

Change-Id: I0dba5f7fb7a671833e47c7fd7c42e77885e26e7a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>