show progress notification (issue #2)

Show uploading progress.

[![](https://camo.githubusercontent.com/84ed6d5893daddbba68ecc528ae4ffd2f5591119/687474703a2f2f692e6779617a6f2e636f6d2f37663736613934396136653163633461306332623033363431663461646165392e676966)](http://gyazo.com/7f76a949a6e1cc4a0c2b03641f4adae9)

This doesn't show real progress. It fake.

### How progress bar increament

* convert image to dataURL : ~30%
* load `/api/upload/easy_auth`: ~80%
* upload finished and open image page in gyazo.com : 100%
Oh I see. It's on web page context, not on background context.