Feature: Simplify usage by supporting new default loop. (#134 by @clue)

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

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

Improve test setup, use GitHub actions for continuous integration (CI) and fix minor typo. (#132 by @SimonFrings and #129 by @mmoreram)