Allow scope to be optional, defaults to "/*"
Support oninstall, onactivate, onfetch in worker
I think this was an early attempt to make cache urls relative, but it isn't complete.

If string urls in the cache constructor/add become worker-relative (which I hope they do), we'll need some kind of Cache constructor-constructor which takes the worker url to use as a base
Merge pull request #57 from jakearchibald/master

Various small changes