Abort installation if version is Python 3

This PR stops the installation process from proceeding if the version is Python 3 (which is not currently supported).

```bash
$ python setup.py install
Sorry, Doga does not support Python 3 at this stage
```

In addition to the above, I've added 'classifiers' (note that Python 3 is omitted there). Fee free to expand the list if you'd like it to support versions prior to 2.7.