Use chrome.storage.local for GistList

This is made slightly more difficult because extensions use an async version of LocalStorage.

http://developer.chrome.com/extensions/storage

I currently cache everything in a POJO for synchronous access and fire-and-forget down to local storage when we add a gist. 

Not sure when the javascript environment on our popup will refresh, so we may need to hook into `Storage.onChanged`. Hopefully we won't.