Tweaks to the new user/role channel format

* When PUTting a user/role and adding new channels to admin_channels,
  setting the associated sequence to 0 will cause it to be set to the
  db's next sequence number.
* TimedSet can be unmarshaled from a JSON array of strings; all the
  sequences will be set to 0. Useful with the above feature.
* Updated the README.