Adds support of co-authors of a patchset merged in openstack

According to the recommendation of the openstack wiki, every
patchset merged in openstack with in the commit message a flag
'co-authored-by' or 'also-by' will be considered for the commits
statistics.

Examples:
  Co-Authored-By: An User <user@os.org>
  Also-By: An User 2 <user2@os.org>

Change-Id: Ia58235c129061942963d34f860230097c64a9828
Implements: blueprint add-support-co-authored-by
Merge "Adds support of co-authors of a patchset merged in openstack"