Make netcdftime.datetime immutable and hashable. Fixes #255.

I went ahead and put together a pull request for the change approved in #255. The ticket approves making netcdftime.datetime immutable so it can be used as a dictionary key. This is my first contribution, so let me of any areas for improvement.

Best,
Daniel
The cleaner way to write this is:

    with self.assertRaises(AttributeError):
        d1.year = 1999