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.

mentioned in commit 1aced1d6

merged