It's now possible to order by associated models. This technically might
have worked before b/c we were parsing JSON sent to `order`, but I'm
pretty sure it wouldn't actually work b/c we never grabbed the actual
model to associate by. Regardless, this actually enables things and adds
tests to prove it.
Note: there's a sequelize bug that's poorly reported but definitely
known where `order` with associated models can fail because the sql
generated doesn't include a join. So, I added docs noting that and a
`test.failing` so that we'll be notified when that bug is fixed and can
remove the note.
