Added most important pieces of metadata for publication

This fixes what's more important about #106.
See [Returning metadata about the document](https://github.com/w3c/specberus/blob/tripu/issue-106-metadata/README.md#returning-metadata-about-the-document) for more information about the way metadata is returned.
So far, the way to refer to the document process was the URI of the process (it's how it's stored in our database and requested by the publication system). Rather than having to do a lookup on the client side, would it be possible to return the URI in the document instead?