Simplify usage by supporting new default loop

[clue]

This changeset simplifies usage by supporting the new default loop.

// old (still supported)
$es = new Clue\React\EventSource\EventSource('https://example.com/stream.php', $loop);

// new (using default loop)
$es = new Clue\React\EventSource\EventSource('https://example.com/stream.php');

Builds on top of reactphp/event-loop#226, reactphp/event-loop#232 and reactphp/http#410