fwd and rwd breaks np

[Fonta]

If you use fwd or rwd, you can't use skip, np, etc anymore because the bot thinks nothing is playing. http://i.imgur.com/RtUUXX9.png

[MinoLiu]

This bug I think the occurred reason should be

_play_entry faster than killed ffmpeg_player callback (_playback_finished)

so _current_player and _current_entry were None.

Created a PR: #28

also fix some other bugs

[TBK]

28 Fixed the issue.