Add unique (event, user) constraint to notification table

Make sure that only one notification exists in the database for each
event and user.
Make sure that notification aren't added twice
Add replay notification script

Add a script which allows to replay all events in order to create
Notification entries to the database which do not exist yet.
Add translator in script

The translator object seems to be initiated by some Pylons foo (I think
it's in the PylonsApp instantiation in config.middleware.make_app),
which isn't executed in the script environment until now.

We might want to put that in the boilerplate code the next time we need
that.
Merge pull request #624 from nidico/feature-replay-notifications

Add replay notification script