Resolve #8 by adding Fennec support

package.json

@@ -8,6 +8,10 @@
     "homepage": "https://addons.mozilla.org/en-US/firefox/addon/decentraleyes", 
     "name": "decentraleyes",
     "id": "id1-BoFifL9Vbdl2zQ@jetpack", 
+    "engines": {
+        "firefox": ">=21.0", 
+        "fennec": ">=21.0"
+    }, 
     "preferences": [
         {
             "description": "Automatically prepend a notice to retrieved documents to signal local delivery.",