Allow to add multiple participation

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

Tests/Controller/EventControllerTest.php

@@ -6,6 +6,11 @@ use Symfony\Bundle\FrameworkBundle\Test\WebTestCase;
 
 class EventControllerTest extends WebTestCase
 {
+    
+    public function testSkipped()
+    {
+        $this->markTestSkipped();
+    }
     /*
     public function testCompleteScenario()
     {