Simplify setup with file instead of Environment Variables

[TechnicalJohn]

-Updated .gitignore for pipenv and secrets.py
-Added example secrets.py file with the contents of what used to be environment variables
-If secrets.py isn't found, offer a meaningful error message pointing in the direction of what to do.

Instead of creating environment variables, why not use a secrets file? I realize there's a small amount of security risk with that, but I would think is negligible versus the ease of setup.

Contributing to Twilio

All third-party contributors acknowledge that any contributions they provide will be made under the same open-source license that the open-source project is provided under.

• I acknowledge that all my contributions will be made under the project's license.