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

Add interface for SQLite
Pass history interface node instead of node.nodeid
Add storage.stop() method for future

History fixes and event support WORK IN PROGRESS