Fix growl registration not sending sickrage update notification registration

Fixes issue #120 
When notifications on update is enabled and notifications via growl are enabled the update notifications are ignored via growl as growl.py is missing line which registers SickRage update as a notification type in growl. SickRage updates and send growl a notification for it but growl ignores it as it is not enabled to be displayed. 
Added line to growl registration which adds common.NOTIFY_GIT_UPDATE as a SickRage notification type.
New registrations/re-registrations to Growl will now correctly enable SickRage update as a notification type.