Improve form behaviour to allow removing of document:

* add button to remove document ;
* fix error when document is removed ;

Resources/config/services/form.yml

@@ -0,0 +1,6 @@
+services:
+    Chill\DocStoreBundle\Form\StoredObjectType:
+        arguments:
+            $em: '@Doctrine\ORM\EntityManagerInterface'
+        tags:
+            - { name: form.type }