Added
- `auth` module supporting authentication via a service account or oauth2
- `exceptions` module with class `OpenAlertError`
- "Overview" section in docs
- classes in `utils` module: `ProjectIds`, `Cast`
- files: `CHANGELOG.md`, `pittgoogle_env.yml`
Changed
- Overhaul the `pubsub` module. Add classes `Topic`, `Subscription`, `Consumer`, `Alert`,
`Response`.
Fixed
- cleanup some issues flagged by Codacy