Fix the "from" address in event confirmation email message.

#6162 

Now the "from" address is fixed:
![selection_255](https://cloud.githubusercontent.com/assets/4497748/3296536/bd80ee1e-f5de-11e3-8f53-60f2851161f9.png)

We need to check if this an required field. If not we need to pull the event author mail when the field is empty.