Simplifying to_dict interface for nested relations

Can now do:

inc = ['id', 'name', 'c1.id', 'c1.name', 'c1.c2.id', 'c1.c2.name']
to_dict(deep=dict(c1=dict(c2)), include=inc)

Which is easier than creating a seperate include_relations dict and
also handles relations of arbitrary depth.