1) Annotation: change "photo" as a reference. photo is a string of file
name in local.
2) FileHelper: update encodeStory decodeStory functions
3) StoryFragmentActivity: can add a picture and store it
perfectly(though we don't need this function right now, it's still
useful when adding a photo manually in local and publish it to the
webserver)
4) Update OfflineStoriesActivity onResume function