IE8+ fires onstorage handler in window that performed storage

[benvinegar]

In Internet Explorer, when UniqueModel syncs to localStorage, IE fires an onstorage event in the same window. We're listening for that event, and we read back the data we just saved, and redundantly update the memory cache.

The evt object includes a url property, which is the url of the window that triggered the event, but we can't use that to reject the event because you could have two windows with the same URL. We'll probably need to include some kind of meta information in the localStorage record about which context created it.