Add possibility to hide empty value in customfield group view rendering

A new parameter is defined :

```
chill_custom_fields:
    show_empty_values_in_views = true|false
```

A new method is added to CustomFieldInterface: `isEmptyValue`. To ease
the dev of new classes, an AbstractCustomField class is created, which
implements the most commons function (currently, only isEmptyValue).

A new Twig Filter is added: `chill_custom_field_is_empty`

The twig filter `chill_custom_fields_group_widget` has a new possibility
in array option : `show_empty`. Default to
chill_custom_fields.show_empty_values_in_view. May be forced by
true/false.

DependencyInjection/ChillCustomFieldsExtension.php

@@ -33,6 +33,8 @@ class ChillCustomFieldsExtension extends Extension implements PrependExtensionIn
         
         $container->setParameter('chill_custom_fields.customizables_entities', 
                 $config['customizables_entities']);
+        $container->setParameter('chill_custom_fields.show_empty_values', 
+                $config['show_empty_values_in_views']);
     }
     
      /* (non-PHPdoc)