Keep use of ModelMarshaler only where really needed

[otsakir]

ModelMarshaler is a component converts data models to json and vice versa. It has some type adapters created by default for handling some special RVD models in a special way. However it's not needed in all places.

We should remove the dependency to ModelMarshaler in *RestServices. It will reduce coupling a little more.

[otsakir]

This also relates to #35 which contains that involved refactoring marshaller class.