Simplify usage by updating to new default loop and making Connector optional

[clue]

This changeset simplifies usage by supporting the new default loop.

// old (still supported)
$proxy = new Clue\React\HttpProxy\ProxyConnector(
    '127.0.0.1:8080',
    new React\Socket\Connector($loop)
);

// new (using default loop)
$proxy = new Clue\React\HttpProxy\ProxyConnector('127.0.0.1:8080');

Builds on top of reactphp/event-loop#232, reactphp/socket#260 and reactphp/http#410
Refs clue/reactphp-ssh-proxy#27