Bug 1093219 - m3u parsing

In addition to a constructor, which will be useful when the user creates new playlists, I suspect you'll also need to write a factor method like Playlist.parse(filename). I imagine that this function would take a filename and return a Promise, and would then use device storage to get the file, and use FileReader to read the content of the file as a string, and then call your load() method and resolve the promise.