adding a proposal for composition of models

`"embed": ["Pet"]` from the `Dog` model copies all the properties of `Pet` into `Dog`.

this mechanism does not provide inheritance, just [DRY](http://en.wikipedia.org/wiki/Don't_repeat_yourself). `embed` just copies the properties of `Pet` into `Dog` so they exist on the wire when you send a `Dog`, but you don't have to re-type everything in `Pet`