Provide Response::<Fresh>::deconstruct to deconstruct a Response.

This allows downstream frameworks by-value access to these fields, letting
them create their own Response abstractions out of hyper's Response.

This is, for instance, needed in Iron to create Iron's Response object, as it
will have to move the Headers out, later putting them back and writing to the
Response.

Requires #19 and will be rebased when/if it is merged.