Moved version info to version.py file

Instead of having the package version in both `setup.py` and `hn/__init__.py`, I think it's better to have it in a single file. I've always liked the way the version was managed in the [rq](https://github.com/nvie/rq/) project. So I've added a `version.py` file in the hn/ directory. Both `setup.py` and `hn/__init__.py` use the value in this file.