Clearing localStorage causes angular-cache to become out of sync

[jmdobry]

See #36

Angular-cache needs to somehow be able to detect when another party has modified angular-cache's items that are saved in localStorage and either fix the problem or modify its own internal state to reflect the changes.

[jmdobry]

The most angular-cache can do is continually sync its own internal state to localStorage, including during reads. That way, even if localStorage somehow gets wiped out with angular-cache is still running in the browser, angular-cache will eventually become consistent with localStorage.