The participationController accept a new parameter : `persons_ids`,
which must receive a comma-separated list of person ids. A participation
will be create for all those peoples.
The `new` and `create` actions does not allow to receive both
`person_id` and `persons_ids`.
Tests are added.
ref #6
Warning : this commit require to rename some table column, but the
original Version file has been update.
If you do not care to loose your date, you may run :
```
php app/console doctrine:migrations:execute --down 20160318111334
php app/console doctrine:migrations:execute --up 20160318111334
php app/console doctrine:cache:metadata #(if you have a cache for
doctrine meta date)
```
close#20
