Initial support for sound localization

If a sound is hearded, try to find the corresponding speaker and set Human.speaking=true
Else, a 'virtual' human called 'unknown_speaker' is added.

Not tested yet
Publish faces in different frame than CameraTop

This is important because we still publish the transform to a face even
if the face is not visible anymore. If the transform is bound to the head
the human position will be useless as soon as the robot moves the head.

So it may be usefull to publish the human pose in a 'fixed' frame like odom.

It does not work well yet because of TF transformation issues, related to
timestamps.