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.

restructure use of types

Example custom enumerations

Conflicts:
	tests/tests_server.py

* for endpoint with port == 0, read the port and hostname from the created server scoket

this sets the port number from the created socket, possibly this change could trickle up to the actual server in the endpoint name

* fix copy paste error

Added tests. Property extended with datatypes. PEP8. Added "get_variables" and "get_methods" for nodes.