Dtd specification

Created DTD specification for HDF export feature
No, no, no - as I said before, don't add images like this, giving an own attribute for each image. This is a very rigid structure and prevents clients from adding additional images if needed. Use a generic '<image>' element and attributes to identify their meaning:

```xml
<images>
   <image rel="body-front" url="..."/>
   <image rel="body-side" url="..."/>
   <image rel="body-back" url="..."/>
</images>