0.5 - Shutdown handling (and shutdown signal)

v0.5

New features

Adds the shutdown method to the scheduler. This will remove and stop all tasks.
There's also a set_shutdown_handler to run an async task after the scheduler was stopped. Conversely, remove_shutdown_handler removes the shutdown handler.

signal feature flag

Optional. Adds shutdown_on_signal and shutdown_on_ctrl_c to the scheduler.
Both shuts the system down (stops the scheduler, removes all the tasks) when a signal
was received (with ctrl+c a specific one).