Simplify usage by supporting new default loop

[clue]

This changeset simplifies usage by supporting the new default loop.

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

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

Builds on top of reactphp/event-loop#226, reactphp/event-loop#229, reactphp/event-loop#232, reactphp/stream#159 and reactphp/socket#260