Add setup.py

[bassosimone]

With this file committed, we could install neubot-runtime from inside another repository by following this simple procedure (and without having to keep a forked copy of the sources):

virtualenv VENV
source VENV/bin/activate
pip install https://github.com/neubot/neubot-runtime/archive/stable.zip

Tested locally with a different pip install command tailored to use this branch rather than stable.

[bassosimone]

(We can ignore the failing travis check: it fails because no .travis.yml is present in the stable branch and hence the repository is treated like a ruby repository)