Fix/gh 734 cleanup audio buffering

Testing this :>

From what I remember when I poked around 

``` python
if percent < 10 and not self._buffering:
    self._playbin.set_state(gst.STATE_PAUSED)
```

Is called multiple times, which consequently breaks internal playbin state.

Test case:
- using ncmpcpp add remote http file 
- start playing
- mopidy state is paused,you can hear sound playing, ncmpcpp will crash
- http client last state == paused, playing is never emitted

Tested on updated ubuntu 14.04