Fix the "from" address in broadcast message.

#4904 

This PR fixes the broadcast message "from" address in the sent emails. This was previously fixed but only took care of the case the address was changed in the event's settings. 

Now the broadcast message is from the correct address (weather set or not in the event settings):
![selection_399](https://cloud.githubusercontent.com/assets/4497748/3669221/fe161f22-122f-11e4-8de4-48ebf3fe2e1c.png)

@ran81 Will the event_author_email always be set?