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/gmail/v1/gmail-api.json

@@ -1,11 +1,11 @@
 {
  "kind": "discovery#restDescription",
- "etag": "\"ye6orv2F-1npMW3u9suM3a7C5Bo/u55VLYY4MP7i0aXvJvy-8Bykhic\"",
+ "etag": "\"ye6orv2F-1npMW3u9suM3a7C5Bo/5025FW69eRcOdd_mm4_PTDhGzHE\"",
  "discoveryVersion": "v1",
  "id": "gmail:v1",
  "name": "gmail",
  "version": "v1",
- "revision": "20150303",
+ "revision": "20150313",
  "title": "Gmail API",
  "description": "The Gmail REST API.",
  "ownerDomain": "google.com",
@@ -1333,6 +1333,12 @@
        "httpMethod": "POST",
        "description": "Directly inserts a message into only this user's mailbox similar to IMAP APPEND, bypassing most scanning and classification. Does not send a message.",
        "parameters": {
+        "deleted": {
+         "type": "boolean",
+         "description": "Mark the email as permanently deleted (not TRASH) and only visible in Google Apps Vault to a Vault administrator. Only used for Google Apps for Work accounts.",
+         "default": "false",
+         "location": "query"
+        },
         "internalDateSource": {
          "type": "string",
          "description": "Source for Gmail's internal date of the message.",