[chrome.app.window] Added getAll()

Since there is only one `AppWindow`, the `chrome.app.window.getAll()` function simply returns it.
And with that patch and a simple fix at https://github.com/kaepora/miniLock/pull/113, we get miniLock to launch on mobile ;)

![screenshot_2014-08-07-10-08-53](https://cloud.githubusercontent.com/assets/634478/3839554/93e2a0d2-1e12-11e4-93ae-1998ce39c1ac.png)

BUG=#259