Base for Vaadin RPC Offline
Initial version with offline support

Contains:
* Offline detection + online/offline events
* DataStore which either caches (local storage) updates or uses rpc to send them immediately
* DataStore which caches (local storage) all addresses
* OfflineRedirector which handles transitions between "/" and "/offline/"

Missing:
* Cache manifest
* Header in offline mode (vertical/horizontallayout can't be used on client side)
* Sensible offline UI (save button seemingly does nothing etc)