Add oss4sink to the music-preview-player

[Mateusz Stachowski]

I'm using Open Sound System on Ubuntu and I always remove PulseAudio and ALSA.

When I try to play music found on SoundCloud in Dash I don't get any sound output, even the button doesn't change it's state. The music preview simply doesn't work in this case. Both versions of Gstreamer 0.10 and 1.0 have support for OSSv4 it's in libgstoss4audio.so and it's available in Ubuntu packages gstreamer-plugins-good.

I've tried adding it on my own but I didn't knew exactly how so I ended up with replacing "pulsesink" in "player.vala" with "oss4sink" and now the music preview works in Dash.