Clean up ExtensionObject handling

Remove downcast_extobject and ExtensionObject class.
All ExtensionObject members are now automatically converted
to objects of correct type. ExtensionObject members are now
initialised as None.

More tools

Now it only supports reading raw values.
Other modes will be added in future versions

Tools

Performs OPC UA browse and prints results.
For example:
tools/ls.py opc.tcp://example.org:4840/OpcUa "i=85"

Performs OPC UA discover and prints results.
For example:
tools/discover.py opc.tcp://example.org:4840

In get_endpoints(): don't set ProfileUris (get all available endpoints)
and don't set LocaleIds (the value was wrong anyway).
In get_server_endpoints(): disconnect socket in the end.

add script for generating debian package