Add item caching.

This is a first attempt to implement item caching and resolve issue #2.

The Drive API is polled every 10 minutes to check if the largestChangeId has changed. If it is the case, the item list will be refreshed. The items are stored in local storage and are loaded on startup if the change id is still the same.

I do not have much experience with Google APIs en Chrome extensions. Feedback is welcome!