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
This feels like a bit of a hack to me, but I suppose it works. What about using private attributes (e.g., `_year`) hidden behind read-only properties?