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

main.js

@@ -68,6 +68,7 @@ function createWindow () {
     query: {
       locale: locale,
       version: package_json.version,
+      buildExpiration: config.get('buildExpiration'),
       serverUrl: config.get('serverUrl'),
       NODE_ENV: process.env.NODE_ENV
     }