WIP serviceworker storage prototype
A first implementation of jIO serviceworker storage. I use it with the serviceworker.js file I put into tests. A document is a cache (there can be multiple), files are attachments.
The storage just implements the jIO api using postmessage, the serviceworker file handles the incoming events and interacts with cache (not indexeddb). It does not hook into fetch to override the default way of requesting files, because I only use it as a pure storage, not a server replacement.
It's missing tests and I don't know which jslint rules to follow. I'll add those when feasible.