Parsing and rendering for more relation types

This builds on pull request #4

- Updated test/model.py Person with additional relations `favorite_post` (nullable ForeignKey) and liked_comments (forward ManyToMany relation)
- Add test serializers, views for "Maximal Person" with new relations
- Handle null ForeignKey as HyperlinkedRelatedField and PrimaryKeyRelatedField with tests
- Handle ManyToManyField as PrimaryKeyRelatedField with tests