Improved Date

For `Date::date_time`, `Date::date_format` and `Date::time`.

Now you can use them as:

```php
Date::date_format($post->created, DateTime::ISO8601); // 2013-11-18T16:48:48+0000
Date::date_format($post->created); // Get format form config/site => date_format
```

The same goes for other methods