signal: add onsubscribers

Change-Id: Ib81e85bd218cedc52aa64ddb2169075b18c8efe4
Reviewed-on: http://gerrit.aldebaran.lan/45495
Reviewed-by: pdaouadi <pdaouadi@aldebaran-robotics.com>
Tested-by: pdaouadi <pdaouadi@aldebaran-robotics.com>