Some users may want to extend the REQ/REP protocols similar to how REQ/REP extends XREP/XREQ.  Exposing the necessary plumbing to do that more effectively.

For a longer discussion of this change, see http://www.freelists.org/post/nanomsg/nn-req-extension

Signed-off-by: Drew Crawford <drew@sealedabstract.com>
Merge pull request #264 from drewcrawford/extendable-reqrep

Extendable req/rep