Http-types

Add `HttpExchangeWriter.to_dict()` and `HttpExchangeWriter.to_json()`.

- Add a version requirement on python-dateutil (19).

- Fix bug in inferring path from url without path (16).
- Handle multi-valued query parameters in path building (17)
- Avoid depending on native C code (18).

- The `req` and `res` properties has been renamed to `request` and `response`, both in the JSON format and on `HttpExchange`.
- When reading JSON, only one of `path` or (`pathname`, `query`) properties is now needed - the missing fields are reconstructed from the other ones.
- Add `timestamp` properties to requests and responses.