improve qi::path doc

and add a test about finding directories

Change-Id: Ieaed604cc1da4363a81f473d6bc3ac1d649ae754
Reviewed-on: http://gerrit.aldebaran.lan/26618
Reviewed-by: dmerejkowsky <dmerejkowsky@aldebaran-robotics.com>
Reviewed-by: jfremy <jfremy@aldebaran-robotics.com>
Reviewed-by: cgestes <gestes@aldebaran-robotics.com>
Reviewed-by: sbarthelemy <sbarthelemy@aldebaran-robotics.com>
Tested-by: sbarthelemy <sbarthelemy@aldebaran-robotics.com>