Code cleanup

[maxmouchet]

• [x] Remove passenger store and associated CLI flag (unused feature, may be re-implemented later if needed)
• [ ] Separate offline/online tests (use pytest markers ? http://doc.pytest.org/en/latest/example/markers.html)
• [ ] Separate CLI repo (locomotive-cli ?)
• [ ] Merge mypy and isort config. to pyproject.toml when supported
• [x] abstract -> base, multiple inheritance for features supported by the clients
• [ ] More tests, increase coverage
• [ ] Change logo (tunnel), update README.md (Julia style)
• [ ] Multithreaded clients
• [ ] Proper date formatting (babel, pendulum, ?)