Add response count to survey object

I'd like to have the responseCount available in the survey object. It'll make it easier to do things like give a survey overview. I initially tried adding a `?details=true` param to trigger the count, but I don't think that's needed -- it's a fast, indexed query.

Ideally, count would also be available in the list of surveys, along with "most recent response", but I don't know the best way to do that with async queries. Would probably use the `async` lib.