option added to disable automatic indexing

This is the first and easiest solution I was able to come up with (https://github.com/mkly/Data/issues/32).

I don't like the fact that I have to call `enableIndexing` before I can call `reindex`:

```php
$data = new Data;
$data->dtID = $dt->dtID;
$data->disableIndexing();
$data->Insert();

$fieldName = 'name';
$fieldValue = 'test';
$dak = DataAttributeKey::getByHandle('data_' . self::DATA_TYPE_HANDLE . '_' . $fieldName);
if (is_object($dak)) {
    $data->setAttribute($dak, $fieldValue);
}
$data->enableIndexing();
$data->reindex();
```