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.
I think it will be confusing to list the keys of the groups to access the WG URLs, and possibly not trivial to access one of them (as opposed to list all of them). I might be missing something here, but since this is a list of groups, I would suggest something along those lines:

``` javascript
groups: [
  {
    homepage: 'http://www.w3.org/XML/EXI/',
    name: 'Efficient XML Interchange Working Group'
  }, // ...
]
```

(Note that, if we want to correlate to our DB, I am not sure if the `name` should instead be called `fullname`, `alphaname`, ...)

FYI, the publication system does not need the name and cannot really use it, because it can differ between 2 documents (for instance, @@@). In fact, the publication system currently fetches the UIDs of all working group matching the homepage URL and proposes the webmaster to choose which actual deliverers want to publish. This secret heuristic-based knowledge has been transmitted by webmasters to their successors for centuries and now the reign of the machines is putting these ancestral traditions to an end.
This issue is similar to the issue we had with the editors. Therefore, we can either require the document to provide the WG ID as an additional metadata (like for the editors), or make sure the WG homepage URLs are unique in the database. I prefer the latter option. We can also decide to not publish such documents...