Features:
- issue 140: expose internal constants to allow changing default
timeouts and polling intervals.
- issue 141: add PID of expired process to `ProcessExpired`
exception. Credits: rsiemens
- Add `pool` decorators parameter allowing to provide a `PebblePool`
as executor for the decorated functions.
Fixes:
- issue 137: reset `SIGTERM` default handler to child processes.
- Include `concurrent` and `asynchronous` modules as top-level
packages.
Improvements:
- Improve type hinting logic. Credits: emiliadecaudin , ccazabon
- Refactor internal logic: remove code duplication.