* 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)

 * replace int_to_* functions with a smarter enum class
 * consolidate WriteAccessMask enum and WriteAccess enum

* Add instantiate support for data types and methods

* Fix for #260

* Allow the import of methods in xml imports

* Adds unittest for testing method importing

* Use server namespaces uri to identify namepsace index for node imports (2)

* Fixes test failures / Add unittest

* Add unittest (2)

Instead of using our own solution for skipping test, we use
unittest.skip. This has the additional benefit that it works
with the automatic test discovery in Python >=3.2.