[MERGE] Enable multiple accounts

Progress:
- [x] Fetch todos from multiple accounts in background
- [x] Wire new background script with badge behavior and caching method
- [x] Handle multiple accounts in Angular code
- [x] Update AngularJS (v1.2.7)
- [x] Bump new version (v3.2.0)

Known issues:
- Korean and Japanese fonts are never applied

Notes:
- Background script has been completely rewritten
- jQuery is no longer required in background
- Underscore.js version is now 1.5.2
- Use `chrome.storage.local` for caching instead of `localStorage` (no more `stringify` or `parse`!)
- Remove most of API calls from the popup (popup lifetime is too short, background tasks should handle them)
- Clean dead code
- It was poorly coded by me ten months ago
- Authentication phase is initiated by user (no more closing/opening tabs thrown from nowhere)
- Embed Korean fonts in app package
- Hovered icons are included in CSS as DataURI: no flickering on first hover

For the next next release...
- Apply [BEM notation](https://github.com/csswizardry/CSS-Guidelines#naming-conventions) to improve CSS readability and reduce nesting level
- Get rid of jQuery
- [Moving from Underscore.js to Lo-dash?](http://lodash.com/benchmarks)
- Implement [rich notifications](http://developer.chrome.com/extensions/notifications.html) (`webKitNotifications.createHTMLNotification` has been deprecated)
- FOUC happens "sometimes"
- Fix tests