Introduce pip dependency management via pip-tools

This resolves #2 (closed) by introducing a requirements.in containing the formerly mentioned dependencies from the Installation section in the README.md. This .in-file then can be compiled to a requirements.txt with pinned versions, which are known to work as exepected, whenever desired. If this goes too far for you upstream maintainers, I would happily just replace the wrong urlparse dependency by anything or remove it. Just let me know. Upstream contributors could as well commit and push to this branch directly.