* 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

changed the function bytes to the function encode for python2 compatibility

Support providing objecttype for the method add_object of Node

new_historized_event now takes list of event types as an argument
function to get list of fields from event types moved into SQL  module
Minor doc string changes

Cleanup examples and add an example for historizing a data value

Match changes in node.py

accept subscription to event type list

Must  be reviewed, do not merge

Implement Subscription Supports List of Event Types, History Update
again after all the changes to events.

fix, renaming and restructure of event stuff