Force ordering by created at

Sometimes my listing as jsonstest would fail for no obvious reason.
Turns out DataMappers `all` method doesn't enforce ordering by default.

We're telling data mapper to order results by the `created_at` field of
the `Overheard` model; and to put the largest (i.e. most recent) at the
top. [Documentation](http://datamapper.org/docs/find.html) (Search for
"Specify the order")
Extract `requesting?` helper

This provides a tiny bit of syntactic sugar that makes our routes look
even more like human speak (hopefully!)

Syntactic sugar is when we make a change primarily for aesthetic
purposes. It's not a bad thing, but it's not something I recommend
optimizing for.