Add add-on id to manifest.json

So that Firefox can use the storage.sync api.
This raises a warning in Chrome:
    "Unrecognized manifest key 'applications'."

manifest.json

@@ -5,6 +5,11 @@
   "manifest_version": 2,
   "description": "Speed up, slow down, advance and rewind any HTML5 video with quick shortcuts.",
   "homepage_url": "https://github.com/codebicycle/videospeed",
+  "applications": {
+    "gecko": {
+      "id": "{7be2ba16-0f1e-4d93-9ebc-5164397477a9}"
+    }
+  },
   "icons": {
     "16": "icons/icon16.png",
     "48": "icons/icon48.png",