add REST documentation for mediacore module

Something else to consider is that there are some models that are used in multiple modules. For those models, I created a "stub" `rest.js` in the `oae-util` module. So there's already a precedent for moving models to a separate file. I'm not real happy with the current approach because there's no way to know whether an endpoint's models are defined in the local file or in the common file; you have to look in both places. It would make it easier to find the documentation if **all** models were in the same place. Of course, the problem with that approach is that it results in one big file and, other than alphabetical, there's not a great way to organize it.