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.
Instead of "the immediately previous version", I would use either "the most recent previous version (in case multiple are shown)" or "the first displayed previous version (in case multiple are shown)". Note that these two statements have a different meaning and I am actually wondering which one is currently applied (it should be the "most recent" one, IMO).