Feature: Allow for cache adapter injection (#38 by @WyriHaximus)

$factory = new React\Dns\Resolver\Factory();

$cache = new MyCustomCacheInstance();
$resolver = $factory->createCached('8.8.8.8', $loop, $cache);

Feature: Support Promise cancellation (#35 by @clue)

$promise = $resolver->resolve('reactphp.org');

$promise->cancel();