[Feature] Saving and loading task list to disk

Fixes #10.

- Saving using a fake link with download attribute.
- Loading using FileReader API, which may not be available on all browsers.
- Using a custom directive to isolate the link file loader.
- Created taskPersistence service to decouple file loading and saving.

Details: [http://stackoverflow.com/q/24005073/147507](http://stackoverflow.com/q/24005073/147507)