Core media

This endpoint must be able to both link an existing media, and create a new one:
http://docs.cmcatalog.apiary.io/#post-%2Fworks%2F{workID}%2Fmedia{%3Ffields%2Cinclude%2Cannotations}

This kind of body links a work:

    { "href": "https://catalog.elog.io/works/5396e592d7d163613d7321ee/media/63613d7321ee5396e592d7d1" }

The href must be a media URI for the catalog baseURL.

Thus, we also need an additional method in core: `core.addMediaToWork`.  It should avoid duplicates (attempting to link an already linked media is a ok, it just doesn't result in any changes to the work).