#274: Warn-code registry

<http://trac.tools.ietf.org/wg/httpbis/trac/ticket/274>

I think we have two options here:

1) establish a warn-code registry
2) disallow definition of further warn-codes (i.e., deprecate extensibility here)

Thoughts? 

Regards,

--
Mark Nottingham   http://www.mnot.net/