Allow passing format string for log date format.

Sometimes you want something else than ISO 8601 date. This patch allows you to pass date format string when you initialize logger. For example something like:

```php
$app = new \Slim\Slim(array(
    "log.writer" => new \Slim\Extras\Log\DateTimeFileWriter(array(
        "path" => "./logs",
        "name_format" => "Y-m-d",
        "date_format" => "Y-m-d H:i:s",
        "message_format" => "%label% [%date%] %message%"
    ))
));
```