extras: add script and config for generating "Who wrote GlusterFS"

This script and configuration can be used to generate statistics similar
to the "Who wrote <linux-version>" series available on http://lwn.net.

Example usage for statistics from the initial branchpoint of 3.4 to the
current development of the release-3.4 branch:

 $ extras/who-wrote-glusterfs/who-wrote-glusterfs.sh \
       v3.4.0alpha...origin/release-3.4

By default the statistics will be calculated over the currently checked
out branch.

Change-Id: Ie7b8b655c50dcb14257e42599e2f89642c8b5b42
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/5912
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Tested-by: Amar Tumballi <amarts@redhat.com>
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-by: Anand Avati <avati@redhat.com>