progress on the DRM

Spent the evening refactoring the Document Rolon Model. The factory now instantiates ElementModel when it doesn't recognize the role, which is great for modeling. The next step will be to drop those subclasses which simply contain constructors and then to modify the plugin method in the factory to associate a model element class with every role. I expect to be able to do all this tomorrow morning.

That then lays the necessary groundwork for XML => DRM binding which I might be able to complete tomorrow as well.