Chrome packaged app

This contains a Chrome packaged app for webwriter. I wouldn't merge this into master as it overrides `save`.  Instead of saving to `localStorage` it uses the [chrome.fileSystem](http://developer.chrome.com/apps/fileSystem.html) api to open and save to the filesystem. That is the main difference with my fork. There's also some minor stylistic changes, like a custom scrollbar (the default scrollbars look terrible on a Chromebook).

I only send this pull request in case you want to "own" this branch and release it into the Web Store. There is a bug in Chrome 29 (current stable branch) that makes the `caretRangeFromPoint` you're using not work in packaged app, but it's fixed in Chrome 30. So I wouldn't publish this until whenever it's fixed in stable.

If you don't about packaged apps or Chromebooks just let me know and I'll be the publisher. Either way, great project!