Add count responder to optionally provide the number of resources in a response header

[lserman]

Not sure how we should determine when clients want the count. ?count=true? A request header?

Anyway, when they do want it, we should just stick an X-Count response header with the # of resources being returned. It's useful whenever the client side has to change their UI based on if theres 0, 1, or many items.

We should also consider ONLY providing the records count and not providing the actual serialization of those records (can be done by overriding to_format)