Deliver plugin as a python package

[maxbrunet]

• Add setup.py and related files
• Move Python module under airflow_exporter package
• Add (commented out) deployment to PyPI via Travis CI
• Update installation instruction in README.md

I chose airflow-exporter/airflow_exporter as package name, but it's open to change. The tests should also be updated to use an installed package (pip install -e . for development and pip install . for CI tests).

Thank you for this nice plugin.

Closes #44

[elephantum]

@maxbrunet great job! We'll test it shortly

[elephantum]

@maxbrunet do I understand correctly, that Travis tests do not use this distribution by package?

[maxbrunet]

@maxbrunet do I understand correctly, that Travis tests do not use this distribution by package?

Yes, I'll work on that if I have time, else I'll leave it to you

[maxbrunet]

Hi @elephantum, I just pushed the updated tests, I took the liberty to do a lot of refactoring. A few things about it:

• It only tests Airflow 1.10.3, since it's the only supported version (for now)
• It uses the matrix feature to easily test more versions (everything should be configurable via environment variables)
• The tests environment is self-contained in docker-compose.test.yml
• I added a docker-compose.yml for development. There, it doesn't really install the package, but mounts the module directly in the user's site-packages (I haven't been able to make pip install --editable work inside the container while mounting the local directory)
• I initiated a CONTRIBUTING.md file to document how to run the dev and test environments
• The AIRFLOW__WEBSERVER__RBAC environment variable is passed to tests container, if ever you need to check if RBAC is enabled or not from there

Hope you'll like the suggested setup.

[elephantum]

@maxbrunet Incredible job! I like your approach it's very elegant and clean.