- configuration(): use attribute_to_value() instead of content_to_set() which is deprecated
- transform.js:
  - remove content_to_set()
  - document attribute_to_value() and value_to_attribute()
  - rewrite attribute_to_value and value_to_attribute() using Alter instead of Compose
  - export XS.Attribute_To_Value and XS.Value_To_Attribute
- test/xs_transforms.coffee: add test for value_to_attribute()