Add async logic

This release allows you to do async concurrent requests. You will need an API key to do these and not be rate limited

$api = new \XIVAPI\XIVAPI();
$api->environment->key('');

// using concurrent
use GuzzleHttp\Promise;

$promises = [
    'item1' => $api->async()->content->Item()->one(1675),
    'item2' => $api->async()->content->Item()->one(5),
    'item3' => $api->async()->content->Item()->one(2546),
];

$results = Promise\settle($promises)->wait();
$items = $api->unwrap($results);

print_r( $items->item1->Name . PHP_EOL);
print_r( $items->item2->Name . PHP_EOL);
print_r( $items->item3->Name . PHP_EOL);