Generate ico and icns from png

`npm run icon-gen` builds icon files from png files in the images
directory, outputting to build/icon.{ico,icns} (the expected location
for electron-builder).

// FREEBIE

package.json

@@ -26,7 +26,8 @@
     "grunt-gitinfo": "^0.1.7",
     "grunt-jscs": "^1.1.0",
     "grunt-preen": "^1.0.0",
-    "grunt-saucelabs": "^8.3.3"
+    "grunt-saucelabs": "^8.3.3",
+    "icon-gen": "^1.0.7"
   },
   "scripts": {
     "test": "grunt test",
@@ -34,7 +35,8 @@
     "start": "electron .",
     "pack": "build --dir --em.NODE_ENV=$NODE_ENV",
     "dist": "build --em.NODE_ENV=$NODE_ENV",
-    "release": "build --em.NODE_ENV=$NODE_ENV"
+    "release": "build --em.NODE_ENV=$NODE_ENV",
+    "icon-gen": "icon-gen -r -t png -m 'ico,icns' -n 'ico=icon,icns=icon' -i ./images -o ./build"
   },
   "build": {
     "appId": "org.whispersystems.signal-desktop",