Add a service worker

[tigerhawkvok]

Service Workers:

http://www.html5rocks.com/en/tutorials/service-worker/introduction/

This will enable prompts to copy to the home screen, eg:

• http://www.androidpolice.com/2015/03/14/chrome-beta-v42-includes-new-prompt-for-adding-web-apps-to-your-home-screen/
• http://arstechnica.com/gadgets/2015/03/chrome-for-android-to-push-web-app-adoption-with-add-to-home-screen-popup/

So the worker itself doesn't need to do much at all.

Blocked on #31

[tigerhawkvok]

MOAR POLYMER:

https://github.com/PolymerElements/polymer-starter-kit#service-worker

https://elements.polymer-project.org/elements/platinum-sw?active=platinum-sw-register