Improved support for Namespaces

* GElement implements set_ns_attr()
* xNode and derivatives enter to a maintenace status, in order
  to be deprecated in future, some tests haven't been added
  due to its internal complexity, like namespaces support. Is
  better to relay on GNode and derivatives. DOM API will be added
  in future versions to deprecate and remove xNode derivatives.
* Fixes on Unit Tests