SoundSystem lib

New
- Plays WAV, Ogg Vorbis and MP3 files, on top of SoundCloud. The decoders are written
  into the lib: nothing else to install.
- MP3: MPEG-1, 2 and 2.5 Layer III, with exact track length (encoder delay and
  padding from the LAME/Xing header) and ID3 tags (title, artist).
- Ogg Vorbis: stereo, mono and surround; tags read from the file.
- Files can be played while they are still downloading, and entered in the middle
  of a track.
- New API for other mods: LocalAudioFiles. Give it the bytes of a file and it plays
  `local:<id>` links like any other track.

Notes
- Required by SoundSystem 2.1.3 or newer.
- Not supported: MP3 Layers I and II, and Vorbis floor type 0.