gen-art-19

- Add a reference to 6.3.2.1 in the definition of
  "configuration_sequence".
- Add a reference to 6.7 in the definition of "transaction_id".

Closes deanwillis/p2psip-base-master#155
Merge pull request #155 from petithug/gen-art-19

Gen-ART 19 added some forward references