Feature/notifications

- Refactored NotifierManager-EmailActor-EmailExecutorActor interplay
- All HerculesActor now have a member value 'notice' which is a NotifierManager. 
- The NotifierManager has four methods: info, progress, warning, critical and calling each method sends a message to the corresponding "channel"
- Which channel(s) each notification type listens to is configured in application.conf
  
I was thinking not to hog down the server over time in case messages bounces etc. May not be much of a problem in reality though. How about setting num_retries = 0 (= infinite) as default but keep the option to limit the number of retries? I guess a permanently failed message could go to the error log.