Lpipe

- `context` is now a required argument of `process_event()`
- Allow `process_event()` without a `path_enum`/`Enum`. One will be automatically generated based on the `paths` provided.
- Refactor and document exception flow control. All lpipe errors will now inherit from `LpipeBaseException`.
- Move setup and tear-down testing utilities out of `conftest.py` and into `lpipe.testing` so they can be used consistently by all projects implementing lpipe.
- Add example of using moto to mock boto3.

- Allow user to set `default_path`. Doing so will ignore all message structure and pass the entire message to the called function.

- Add check_status to utils

- `utils.get_nested` will attempt a getattr() call if HEAD isn't a dict.

- Add `utils.set_nested` dict helper.

- Add JSON encoder `utils.AutoEncoder` which handles encoding Enums.