Simplify usage by supporting new default loop

[SimonFrings]

This pull request is a follow up for #16. Thanks to @PaulRotmann for investing time into this 👍.

This changeset simplifies usage by supporting the new default loop.

// old (still supported)
$factory = new Clue\React\Mdns\Factory($loop);

// new (using default loop)
$factory = new Clue\React\Mdns\Factory();

Builds on top of reactphp/event-loop#226, reactphp/event-loop#229 and reactphp/event-loop#232.

Resolves and Closes #16.

[clue]

@SimonFrings and @PaulRotmann Thanks for the update, changes LGTM!