Rename previous to activeWorker https://github.com/slightlyoff/ServiceWorker/commit/ac18d602427b7ae5c39d2d635242c3850dd59d28
Incorporate https://github.com/slightlyoff/ServiceWorker/commit/747dba12eb12a9b8f0e10763eeb0f1a3062c5403
Incorporate https://github.com/slightlyoff/ServiceWorker/commit/bbf399ec97b8a6c0ab60246222a34312c7161899
 Incorporate https://github.com/slightlyoff/ServiceWorker/commit/92491669a2427f74f9f081d36e7ffe56b70ff307