Allow passing format string for log date format.

Allows you to initialize logger with something like following:

$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%"
    ))
));
Merge pull request #71 from tuupola/log_date_format

Allow passing format string for log date format.