Add system date to entry xml

Makes system dates available under

```xml
<system-date>
    <created iso="2013-05-08T23:51:48+01:00" time="23:51" weekday="3" offset="+0100">2013-05-08</created>
    <modified iso="2013-05-08T23:51:48+01:00" time="23:51" weekday="3" offset="+0100">2013-05-08</modified>
</system-date>
```