Fixed XmlParser not recognising DateTime objects when dumping to XML

#643

The XmlParser now generates the following xml for \DateTime instances:
```xml
<data>
  <time type="xsd:dateTime">2014-07-23T22:27:17+0200</time>
</data>
```

I'm not sure if setting `type` to `xsd:dateTime` is the right solution, but without setting a `type` attribute we cannot use `fromXML` with the generated xml because we cannot know if a dom node should map to a `\DateTime`.