detail expansions

[geemus]

We commonly include an envelope containing foreign keys for parent objects. To simplify and avoid n+1 queries, it can often be very useful to ask for those envelopes to be expanded to serialize the entire parent object. We've started experimenting with this at Heroku and I wanted to at least note it here and link to a raw doc around it. I want to let the experiment run a bit longer before I settle on it and include it, but wanted to give some early ideas.

Copy of rough spec here: https://gist.github.com/geemus/6b0d59b5813d765c5267