XML importer updates

Choose PolicyId from list of policies

Refactor command-line tools

In ActivateSession parameters, PolicyId must match the Id of needed
UserTokenPolicy of endpoint. Some servers ignore this Id, but some
(like OPC Foundation's .Net UA library) return an error if PolicyId is invalid.

Fixed function add_variable ignoring obj.value when a 0 was parsed from xml.

Updated function _parse_value to support more data types (which are common to python).

Adds pycrypto requirement and corrects server example in README

Fix conversion from FILETIME to datetime

Previously, datetime.utcfromtimestamp() was used,
which is limited to 1970-2038 range on 32-bit POSIX systems.
Adding timedelta to datetime(1601, 1, 1) works better.

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.