New relation type: embedsOne

This is mainly for consistency, as well as enabling validation of embedded objects.

Some other things worth noting:
- model-based properties are now coerced on initialization of the parent model (see lib/model.js line 188)
- embedded models are now validated by default (when saving the parent model - see #136)
- introduced unsetAttribute method (#208)
- unsetting an embedded model sets it to `null` - there's a related issue though: #234