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

Adding the methods necessary for Modifying Subscriptions, make deadband filters and also added support to deadband filter on server side.

default strings and ByteString to None

small renaming, mv more methods to events.py