[feature] Rename $queryString to $query_string and add support for PDO prepare

Rename 
$queryString to $query_string, 
$this->queryString to $this->query_string

for consistently naming variable with underscore

and add support for PDO prepare that returns a PDOStatement object

$medoo = new medoo();
$sth = $medoo->prepare("SELECT * FROM countries WHERE Population > :population");
$sth->execute(array(':population' => 103000));
$data = $sth->fetchAll();

Sorry I don't know why your commit include on my pull request. :smiley: