Pause gstreamer when buffering

When the `playbin2` pipeline is in `PLAYING` state and its buffer is empty the pipeline will consume arriving data immediately. If the source is an HTTP stream this leads to choppy playback. To fix this we pause playing when the buffer is nearly empty and wait for it to fill up until we resume playing. This approach is recommended in the [gstreamer manual][1].

We might want to extract the hard-coded 10% mark and make it configurable.

[1]: http://gstreamer.freedesktop.org/data/doc/gstreamer/head/manual/html/chapter-buffering.html