Build expiration date

Add the buildExpiration config and add it to the renderer's config
object. Use grunt to write the build expiration to
config/local-production.json which will override the default value (no
expiration) in production. Finally, run this grunt task as part of the
build process.

// FREEBIE

package.json

@@ -33,7 +33,7 @@
     "test": "grunt test",
     "lint": "grunt jshint",
     "start": "electron .",
-    "dist": "build --em.environment=$NODE_ENV",
+    "dist": "grunt && build --em.environment=$NODE_ENV",
     "pack": "npm run dist -- --dir",
     "pack-staging": "NODE_ENV=staging npm run pack",
     "dist-staging": "NODE_ENV=staging npm run dist",
@@ -74,6 +74,7 @@
       "package.json",
       "config/default.json",
       "config/${env.NODE_ENV}.json",
+      "config/local-${env.NODE_ENV}.json",
       "background.html",
       "_locales/**",
       "protos/*",