Fix minor issues cause crash

1.Add function to send short Http msg after a song played.
2.When finishing play the list, send Http msg to request new list
3.Start Phonon play() by check Phono state, cause in some cases,
  downloading is slower than playing
4.In startDownload(), check Iterator's empty, valid after checking if
it's end of list, otherwise, Iterator overflow
5.If file has exist, don't download again.