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

* support multiple depths of inheritance for instantiate

* move common methods to common/ua_utils.py

* add method is_child_present to ua_utils.py

* cleanup empty newlines

* Improve implementation of is_child_present

* [is_child_present] avoid break

* Updated examples to reflect the proposed changes in the API

* pdated examples to reflect the proposed changes in the API, fix wrong method

* Updated examples to lateste version of the API including the instantiate

* revert renaming of server-custom-object.py

* Initial refactoring the import iteration routine

* Added logic for sorting nodes and respect theire parent id

* Fixes tests

* Fixes some PEP8 issues

store status code in UaStatusCodeError

This allows the user to handle some status codes differently
from others.

Improve enums

Fix event notifier bugs

Inlude Server and Source timestamps in monitored_items so that it can be retrived in data change handler (#265) (#266)