feat(downloads): alt 'media' handling to allow dls

This also includes documentation to state which methods actually support
media download, and how to achieve that.

Added TODO to not forget we should tell the user how to achieve these
kinds of things.

Fixes #21

etc/api/plus/v1/plus-api.json

@@ -1,11 +1,11 @@
 {
  "kind": "discovery#restDescription",
- "etag": "\"ye6orv2F-1npMW3u9suM3a7C5Bo/q8dY4UIVNRcK1B68dybhQ1dBwWc\"",
+ "etag": "\"ye6orv2F-1npMW3u9suM3a7C5Bo/ORNICSF2Z1YjV7yMXdU4V945lGo\"",
  "discoveryVersion": "v1",
  "id": "plus:v1",
  "name": "plus",
  "version": "v1",
- "revision": "20150122",
+ "revision": "20150303",
  "title": "Google+ API",
  "description": "The Google+ API enables developers to build on top of the Google+ platform.",
  "ownerDomain": "google.com",