Issue parsing SLD TextSymbolizer -> PropertyName

When parsing content like:

```
       <TextSymbolizer>
         <Label>
           <ogc:PropertyName>name</ogc:PropertyName>
         </Label>
```

JSONIX seems to not resolve the value of PropertyName:

![screen shot 2014-12-11 at 15 37 03](https://cloud.githubusercontent.com/assets/319678/5395662/94d9f8b8-814b-11e4-895c-abc7c7117003.png)