Python 3 Compatibility / Better CI

1. Python 3 support. For the love of Jesus.
2. Rev all dependencies including Flask and Twilio.
3. Increase app code test coverage to 100%.
4. Add CI for Py2 and 3 with PEP8 and coverage tracking.
5. Migration to tox for testing.
6. Move test utilities out of requirements and into tox.ini.
7. Beef up README.