Use promises for query() method

[clue]

This PR updates the query() method to use promises instead of accepting an optional callback function. The updated query() method also accepts an array of query parameters for consistency with the queryStream() method (#57):

// old
$connection->query('CREATE TABLE test');
$connection->query('DELETE FROM user WHERE id < ?', $id);
$connection->query('SELECT * FROM user', function (QueryCommand $command) {
    if ($command->hasError()) {
        echo 'Error: ' . $command->getError()->getMessage() . PHP_EOL;
    } elseif (isset($command->resultRows)) {
        var_dump($command->resultRows);
    }
});

// new
$connection->query('CREATE TABLE test');
$connection->query('DELETE FROM user WHERE id < ?', [$id]);
$connection->query('SELECT * FROM user')->then(function (QueryCommand $command) {
    if (isset($command->resultRows)) {
        var_dump($command->resultRows);
    }
}, function (Exception $error) {
    echo 'Error: ' . $error->getMessage() . PHP_EOL;
});

This PR also includes relevant documentation and tests. The old method definition was somewhat inconsistent and under-documented. For instance, it had some limited streaming capability by not passing a callback. You should use the new queryStream() method instead.

Refs #57
Refs #4