implemented notification disable option

https://github.com/globaleaks/GlobaLeaks/issues/647
https://github.com/globaleaks/GlobaLeaks/issues/547
https://github.com/globaleaks/GlobaLeaks/issues/587

this option permit fast temporary notification suspend. 

the logs now may contains these new lines:

    2013-10-03 15:35:25+0200 '[!] Node has Notification temporary disabled'

this happen when a notification scheduler run in a node with notification disabled (log level error, so, it's always displayed)

     2013-10-03 15:33:34+0200 [ ] Switching notification mode: was ENABLE and now is DISABLE

and in the opposite way.