Interactors MAY return headers, message version.

Details:
* Interactors MAY now return extra headers to be added to the parcels.
  But these headers are superceded by the reserved header values
  by the Serf system, and MAY be overwritten by other middleware.
* Reserves the 'version' header field as the semantic version of the
  Domain Object as represented by the message-kind pair in the given
  parcel. This is returned along with other headers.
* NOTE: We want the Serf Interactor API to be really focused on
  the Domain Object, thus the return result is still emphasizes
  kind and the message.
Interactors can return version embedded in the kind.

Details:
* Creates a shorthand way to let Interactor developers return
  the version information within the kind itself, without needing
  to use the extra headers field. A little more succinct.
* The embedded version info is processed by the serfer and moved
  into the proper parcel header 'version' field; updates the
  kind to just be the domain object kind, sans extra info.