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.

More tools

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