API change: most methods now returns owned values

* Change required for GNode series implementation, due to
  use internal pointer managed by libmxl2, then objects
  are created when needed no reference is kept

* All implementations ported to new API on GXml.Node*

* TwDocument.write_document now support any GXml.Document

* Added initial tests for HtmlDocument, actualy failing
  on search elements with given "id"