add REST documentation for mediacore module

I honestly can't recall.

I think the potential confusion is from the Hilary code

    MediaCoreAPI.updateThumbnails(req.body.media_id, function(err) {

which would seem to imply (to me, at least) that `media_id` is part of a body parameter. Lack of experience with express.

@stuartf is looking into how to make the parser accept optional form parameters.

Now I'm not sure if there's any way we can make Swagger support our mapped objects, though. I'll try some additional tests.