Support embedded objects w/ belongsTo relationship

See proposal on discuss.emberjs.com:
http://discuss.emberjs.com/t/ds-activemodelserializer-support-for-embedded-objects-using-belongsto-relationship/3392

Squashed 4 commits:
- Add failing test for serializing embedded (belongsTo) object
- Pass test for serializing embedded (belongsTo) record
- Add failing test for extractSingle with embedded object (belongsTo)
- Pass tests for extractSingle with embedded object (belongsTo relationship)