Added option to return affected rows to DibiFluent::execute

If the return parameter is dibi::AFFECTED return DibiFluent::execute number of affected rows.
Example:
```php
$connection->update('test', $data)->where('xxx=1')->execute(dibi::AFFECTED_ROWS));
$connection->delete('test')->where('xxx=2')->execute(dibi::AFFECTED_ROWS));
```