If node value is type Guid get the value from the child <string> tag.

fixes for opcua-modeler

* ignore BadNoSubscription after deleting the last subscriptions

Spec Part 5, Paragraph 13.8.1 says that outstanding publish responses
should be answered with BadNoSubscription in case the client just
deleted the last subscription.

This can lead to the following exchange:

-> Publish
-> Delete
<- Publish response (BadNoSubscription)
-> Delete response

We did not correctly ignore BadNoSubscription in this case.

* check response of close session request

initial copy_node implementation and cleanup of instanciate

* Implement XML exporter

* Add support for UAObject and references

Exporter can now export regular objects. First attempt at exporting
references added for review. Split some methods so building etree and
dump/write are separate.

* Add reference aliases, add export of object type

* Added UA data type, variable type, namespace uris

* Example for custom namespace uri

simple changes for opcua-modeler

This enables us to catch status codes directly like this:

    try:
        raise UaStatusCodeError(0x80010000)
    except BadUnexpectedError:
        # do something