* Dependency resolution/checking is now working on both sides of the two party trading protocol.
* The commercial paper contract was changed to check timestamping authority identities by name instead of key.
* The trader demo has been rewritten to use the protocol framework, which simplifies the code.
This commit is incomplete: only the seller side currently checks. The code will be refactored out into subprotocols in further commits.
Note that timeouts are currently unhandled.
