Support query with count

[dplewis]

See parse-community/Parse-SDK-JS#868

$query->withCount();
$response = $query->find();
$response['count'] // total number of objects despite limit / skip
$response['results'] // Array of objects

As of PHP 7.1 you can use Array Destructuring like es6

['count' => $count, 'results' => $results] = $query->find();
$count // total
$results // array

[davimacedo]

LGTM!