Simplifiying Helper and twig functions

Now, the CustomFieldsHelper methods and the twig function which render customFields require always a `Chill\CustomFields\Entity\CustomField`.

The possibility to add a slug and an object as argument to those method is abandoned, and was not used in other bundle (nevertheless, the method to catch a customField from his slug was buggy).

Service/CustomFieldsHelperException.php

@@ -26,11 +26,11 @@ class CustomFieldsHelperException extends \Exception
 {
     public static function customFieldsGroupNotFound($entity) 
     {
-        return new CustomFieldsRenderingException("The customFieldsGroups associated with $entity are not found");
+        return new CustomFieldsHelperException("The customFieldsGroups associated with $entity are not found");
     }
     
     public static function slugIsMissing()
     {
-        return new CustomFieldsRenderingException("The slug is missing");
+        return new CustomFieldsHelperException("The slug is missing");
     }
 }