[WIP] Portaudio Integration with a little help from ffmpeg

This PR replaces the Coreaudio layer with Portaudio. For a safe running audio experience we need to put the audio processing into a separate process as the low level callback is a bit fragile and possibly conflicts with Curses.

It works ok for now, but still needs improvement.

Architecture:

```
Soundcloud2000 -> Ruby { ffmpeg  -> float buffer -> portaudio }
```

Problems:
* float buffer for a long song will take to much memory
* skipping forward causes annoying glitches

Next steps:
* native mp3 decoding as ruby extension
* decode frames on the fly
* better skipping
* less memory