move MDM types to micromdm/micromdm (#341)

Previously the github.com/micromdm/mdm library was used, but the code there was a bit naive and had a few issues as it grew to support the full library of MDM Commands. 

All the request types have been moved to `package mdm`.

platform/queue/queue.go

@@ -204,9 +204,9 @@ func (db *Store) pollCommands(pubsub pubsub.PublishSubscriber) error {
 				if err == nil && byUDID != nil {
 					cmd = byUDID
 				}
-				newPayload, err := plist.Marshal(&ev.Payload)
+				newPayload, err := plist.Marshal(ev.Payload)
 				if err != nil {
-					fmt.Println(err)
+					fmt.Println(errors.Wrap(err, "marshal event payload"))
 					continue
 				}
 				newCmd := Command{