Fix: downtime can have no extracomment.
Fix: downtime can have no extracomment.

[scheduler-1] Exception type : <type 'exceptions.AttributeError'>
Back trace of this kill: Traceback (most recent call last):
  File "/home/greg/Documents/Projets/shinken/shinken/scheduler.py", line 677, in retention_load
    b = inst.load_retention_objects(self, logger)
  File "/home/greg/Documents/Projets/shinken/shinken/modules/pickle_retention_file_scheduler.py", line 171, in load_retention_objects
    sched.add(dt)
  File "/home/greg/Documents/Projets/shinken/shinken/scheduler.py", line 230, in add
    f(self, elt)
  File "/home/greg/Documents/Projets/shinken/shinken/scheduler.py", line 209, in add_Downtime
    self.add_Comment(dt.extra_comment)
  File "/home/greg/Documents/Projets/shinken/shinken/scheduler.py", line 215, in add_Comment
    self.comments[comment.id] = comment
AttributeError: 'NoneType' object has no attribute 'id'

kaji-project/shinken,xorpaul/shinken,staute/shinken_package,lets-software/shinken,mohierf/shinken,h4wkmoon/shinken,naparuba/shinken,geektophe/shinken,staute/shinken_deb,baloo/shinken,rednach/krill,mohierf/shinken,gst/alignak,staute/shinken_package,rednach/krill,rednach/krill,Simage/shinken,fpeyre/shinken,Aimage/shinken,geektophe/shinken,tal-nino/shinken,kaji-project/shinken,peeyush-tm/shinken,Alignak-monitoring/alignak,KerkhoffTechnologies/shinken,Alignak-monitoring/alignak,claneys/shinken,xorpaul/shinken,Simage/shinken,KerkhoffTechnologies/shinken,titilambert/alignak,peeyush-tm/shinken,fpeyre/shinken,xorpaul/shinken,KerkhoffTechnologies/shinken,baloo/shinken,mohierf/shinken,h4wkmoon/shinken,fpeyre/shinken,rledisez/shinken,tal-nino/shinken,claneys/shinken,rledisez/shinken,baloo/shinken,xorpaul/shinken,Aimage/shinken,naparuba/shinken,titilambert/alignak,lets-software/shinken,Simage/shinken,kaji-project/shinken,rledisez/shinken,h4wkmoon/shinken,geektophe/shinken,tal-nino/shinken,titilambert/alignak,fpeyre/shinken,staute/shinken_deb,Aimage/shinken,kaji-project/shinken,savoirfairelinux/shinken,xorpaul/shinken,rednach/krill,savoirfairelinux/shinken,ddurieux/alignak,kaji-project/shinken,peeyush-tm/shinken,gst/alignak,claneys/shinken,lets-software/shinken,staute/shinken_deb,geektophe/shinken,h4wkmoon/shinken,savoirfairelinux/shinken,tal-nino/shinken,baloo/shinken,kaji-project/shinken,mohierf/shinken,staute/shinken_package,dfranco/shinken,Aimage/shinken,xorpaul/shinken,staute/shinken_deb,rledisez/shinken,dfranco/shinken,naparuba/shinken,staute/shinken_deb,staute/shinken_deb,mohierf/shinken,rednach/krill,claneys/shinken,titilambert/alignak,tal-nino/shinken,peeyush-tm/shinken,h4wkmoon/shinken,lets-software/shinken,baloo/shinken,peeyush-tm/shinken,ddurieux/alignak,savoirfairelinux/shinken,kaji-project/shinken,Simage/shinken,tal-nino/shinken,staute/shinken_package,KerkhoffTechnologies/shinken,ddurieux/alignak,rledisez/shinken,geektophe/shinken,peeyush-tm/shinken,lets-software/shinken,staute/shinken_package,ddurieux/alignak,Aimage/shinken,savoirfairelinux/shinken,h4wkmoon/shinken,dfranco/shinken,Aimage/shinken,fpeyre/shinken,geektophe/shinken,savoirfairelinux/shinken,h4wkmoon/shinken,claneys/shinken,claneys/shinken,rledisez/shinken,staute/shinken_package,naparuba/shinken,dfranco/shinken,Simage/shinken,ddurieux/alignak,xorpaul/shinken,KerkhoffTechnologies/shinken,Simage/shinken,dfranco/shinken,xorpaul/shinken,fpeyre/shinken,baloo/shinken,h4wkmoon/shinken,naparuba/shinken,mohierf/shinken,ddurieux/alignak,rednach/krill,gst/alignak,naparuba/shinken,lets-software/shinken,dfranco/shinken,KerkhoffTechnologies/shinken,gst/alignak