add REST documentation for mediacore module

Note, though, that we're at the mercy of Mediacore here. If they add new properties to the object that they return, we'll just reflect them in our API. Rather than trying to keep in sync with Mediacore, we should probably just document the properties that we use in the API (which is currently only the `html` property). We can, however, update the documentation from, e.g.

     * @param  {Object}     callback.embedCode          A JSON object with one element, 'html', which is the html code that should be inserted in the page

To something like

     * @param  {Object}     callback.embedCode          A JSON object with at least one element, 'html', which is the html code that should be inserted in the page