Add required packages in the setup.py

If the [`install_requires` option][1] is presented on the `setup.py`, required packages will be automatically installed when running `python setup.py install`, or `pip install greentornado` after you've uploaded this package in the PyPI. This is also useful in development. Try `python setup.py develop`.

[1]: http://pythonhosted.org/distribute/setuptools.html#new-and-changed-setup-keywords