elaborate date conversion

```
Currently complete simple gregrorian dates are converted to a ^^xsd:dateTime. 
Rather use the full range of W3CDTF formats before falling back to the gedcom 
string and don't add time details when it was not there 


```

Original issue reported on code.google.com by `jo3....@gmail.com` on 23 Jan 2013 at 9:18
