Include all data automatically #263

If I have a series of nested models (from SQLAlchemy, for example), it would be nice to be able to RootSchema(include_data=True).dump(root_model). Currently, you have to spell out each and every relationship you want to include, but it's not inconceivable you might want to dump out the whole relationship tree.

My proposed API for this would be setting include_data to True, rather than passing in an iterable.