Flasksimpleauth

Improve debugging experience with `debug4`.

- Improve `demo` with an upload example.
- Fix display of authentication source in `dev` mode.
- Better check parameter types.

For GAS:
- Add `FSA_KEEP_USER_ERRORS` configuration directive to skip handling
user errors and let them pass to the WSGI infrastructure instead.
- Add convenient messages when missing an optional module.
Simplify optional dependencies.
- Log internal error traces as errors.

Others:
- Extend special parameter functions with a parameter holding the name of the expected parameter.
- Add `FileStorage` special parameter to get file uploads.
- Add *manual* table of contents to `README.md` and `DOCUMENTATION.md`.

For GAS:
- improve debugging experience by logging the backtrace
- add installation options for dependencies

- reduce debug verbosity by adding a third level

- reduce verbosity on `current_user`.