Simplify usage by supporting new default loop

[clue]

This changeset simplifies usage by supporting the new default loop.

// old (still supported)
$stdio = new Clue\React\Stdio\Stdio($loop);

// new (using default loop)
$stdio = new Clue\React\Stdio\Stdio();

Builds on top of reactphp/event-loop#229 and reactphp/event-loop#232 and reactphp/stream#159