Add retina popup icon

manifest.json

@@ -10,7 +10,10 @@
     "128": "icon128.png"
   },
   "browser_action": {
-    "default_icon": "icon.png",
+    "default_icon": {
+      "19": "icon.png",
+      "38": "icon76.png"
+    },
     "default_popup": "popup.html",
     "default_title": "Publish this"
   },