Add missing project metadata

In order to publish this project to PyPI, the following metadata keys should be added to setup.py (as keyword arguments to the setuptools.setup() function call):

• classifiers
• description
• license
• long_description
• url
• project_urls

See https://setuptools.pypa.io/en/latest/deprecated/distutils/setupscript.html?#additional-meta-data for descriptions of all but the last of these, project_urls should be a dict of (key, url) pairs where the keys should include: Bug Tracker (a link to the issues page), Documentation (a link to the online docs), and Source Code (a link to the git repo) - this is used to populate the PyPI page with some nice extra links.