This commit is very functional, but WhereClause is not supported yet.
Default history will fail tests because it raises NotImplemented on
event calls.

Event implementation moved to server

Example update and new client-events example

Events fixing

Events implementation proposal

New-style classes

Unless you are doing some hack that requires old-style classes, they should be new-style ones to ensure forward compatibility.

Turn Server into a context manager

Turn Client into a context manager

Allow using Server with the with statement:

server = Server()
# Set up the server
with server as server:
    # Update the server

Allows using client as a context manager for the with statement:

with Client(URL) as client:
    client.do_something()

Added support for Inverse references

Null string instead of string of 0 length for null QualifiedName and LocalizedText
This is what uaexpert does, so it is probably correct

SQL History storage