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.

Templating/Twig/CustomFieldRenderingTwig.php

@@ -76,10 +76,21 @@ class CustomFieldRenderingTwig extends \Twig_Extension implements ContainerAware
                 'is_safe' => array(
                     'html'
                 )
+            )),
+            new \Twig_SimpleFunction('chill_custom_field_is_empty', array(
+                $this,
+                'isEmptyValue'
             ))
         ];
     }
     
+    
+    public function isEmptyValue($customFieldorClass, $fields, $slug = null)
+    {
+        return $this->container->get('chill.custom_field.helper')
+            ->isEmptyValue($fields, $customFieldorClass);
+    }
+    
     /* (non-PHPdoc)
      * @see Twig_ExtensionInterface::getName()
      */