contrib: implement rfc6062 in pjnath

A first try to implement the RFC 6062 into PJNATH.
More explanations into the rfc6062.patch itself.

Note: not ready to be pushed up-stream to pjproject.

Change-Id: I445f0c8a8db03c9a4a5b04d24ef63ff4f711a725
Reviewed-by: Anthony Léonard <anthony.leonard@savoirfairelinux.com>