Implement client side caching

Improve caching strategy via service workers, examples:
- [gulp-sw-cache](https://github.com/yanzhihong23/gulp-sw-cache)
- [sw-precache](https://github.com/GoogleChromeLabs/sw-precache)
- [gulp-sww](https://github.com/arcturus/gulp-sww)

_Part of generic Optimization issue #39_