Add sendNotificationMessage hook

[dmolineus]

This pull request provides a hook before sending a message.

I'm using the notification center in an application which sends a notification when something happens. This notification creates different messages for different persons. For example a productWasOrdered notifcation would send different messages to the product owner, the user who ordered the product and the system administrator.

The product owner has the option to disable such email notifications in his profile. To respect his user settings the hook is required.

Alternatively I would have to create different notifications for each receiver and handle the settings when creating the notification. But this would limit the flexibility which the notification center claims to provide.

[dmolineus]

Another advantage is that the tokens could get enriched using the hook. So when creating the notification I only have to pass the recipientId. A hook would add the member username, email and so on automatically.

[Toflar]

I would add this to 1.3 even though already in RC phase. I don't expect to have 1.4 anytime soon so I think we should accept this. @aschempp, @qzminski: If you agree, I'll merge it.

[qzminski]

I'm fine with adding it now.

[dmolineus]

Would be great if it will be part of release 1.3.0. Be aware that I've already rebased the branch with the changes in release/1.3.0. as I needed the fixes made there to get it installable.

If you want to merge this PR onto another branch, feel free to inform me. I can reopen it.

[Toflar]

each message being sent

[Toflar]

Looks good to me, one little spelling mistake but then I'll merge :)

[dmolineus]

Typo got fixed.

[Toflar]

Thanks for the hint, unfortunately one does not get any notification when a new commit is pushed to a PR :)

[dmolineus]

I know. That's why I also add the comment.