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.

fix __exit__ method of client and server class

Added support for custom datatypes and removed some trailing spaces

* Issue #226

Add server Event for client Item subscription

* Add exemple for Callback

* Add datatype to create_variable

* Add datatype to create_variable, code cleanup

* Support xml address space import with custom data types

* Support xml address space import with custom data types

* small cleanup, add 2 methods in ua_utils.py

* improve and add more tests to string convertions

* update README

* add definition for EvenNotifier and WriteMask, ObjectIdNames and start using them

* update dependency to datetuil

* more use og new Enums, fixes

* DataType cannot always be converted to VariantType

changed _encrypt_password() so it returns (data,uri) as expected by _add_user_auth(). Fix for issue 219 (#220)

xmlimport breaks when UAVariable doesn't have value element (2)

Make History Consistent with Events

do not stop client for protocol error, but try again