fix initStatistics() crash if media has no audio or video(issue 37)
replay do not use seek(0). emit stopped() if avthread finished

TODO:
demuxer can not get EOF packet if use seek(0), but why? (issue 32)
shall we emit stopped() only once?
mark eof packet and not use signal/slot
larger PacketQueue capacity and threshold for large fps
Merge branch 'master' into player