CloudFiles can now request the list of containers for a user in a
specific region by specifying the region on the API call.

Need to support a region specified (immutably) in the configuration. But
then allow the user to override it on a per method basis.

A client now has multiple Transports: one per region. We're not going
to have any of that "one client per region" nonsense.

But this introduces Waaaaay too much code driven by a single test...
Extracted Faraday-specific HTTP semantics into the Transport class where
it probably belongs.
* Extract more from Client#request
* Add an (as yet untested) allowed status codes param to Client#request
* Client#region now gets the value from its config Hash. May be
  approaching extracting a Configuration class wrapping a Hash.
Let's have a useful error message.